I often encounter Catholics who are unfamiliar with the Eastern Catholic Churches. The first questions that usually come up are “Are they really Catholic?” and “Can we receive communion at their churches?” The short answer to both is “yes” and “yes”. Zenit News Service, the Catholic news agency, has a nice Q&A which goes into some more detail:
Q1: Is there a real division/separation between Catholics of the Latin rite and Catholics of Eastern rites? Is a Catholic of the Latin rite debarred in anyway from participating in the liturgy of an Eastern-rite Catholic church? Does a Latin-rite Catholic have to follow a procedure before he can participate in the liturgy of an Eastern-rite Catholic church? — H.W., Port-of-Spain, Trinidad and Tobago
Q2: May a Catholic attend Mass in an Orthodox church? Is not the Orthodox Church schismatic? — E.T., Mairé-L’Evescault, France
A: Since these two questions are related I will take them together.
First, there is no division or separation between the Latin rite and the more than 20 Catholic Eastern Churches. There are, however, many differences and distinctions.
These multiple distinctions give each Church its characteristic identity within the one fold which is the Catholic Church.
The most obvious distinctions are external. Each Church uses a distinct ritual for Mass, the sacraments and sacramentals.
For those Churches where there is a corresponding Orthodox Church (for example, the several Byzantine or Melkite Churches, the Coptic, and the Syro-Malankara), an outsider would be hard-put to distinguish between the two celebrations. One key difference with the Orthodox: The Eastern-rite Catholics mention the Pope in the anaphora, or Eucharistic Prayer.
Compared to the Latin-rite Church, the Eastern-rite Churches differ in their internal organization. This is evident, for example, in the guiding role of the patriarch or major archbishop, the means of selecting bishops, and in some cases the presence of married priests.
None of these differences, however, constitute a separation of faith or of communion with the See of Peter.
Because of this, any Catholic may attend, receive Communion, and fulfill the holy day precept at any Catholic rite.
There is no formal procedure required before attending, but the ancient principle of “When in Rome, do as the Romans do” should be diligently applied. Thus a Latin Catholic who wishes to attend one of these rites should acquaint himself with the basic practices and demands of the rite and adapt himself accordingly. For example, most Eastern rites remain standing for most of the celebration and do not kneel for the consecration; a Latin should respect this tradition. Some rites have stricter fasting rules before receiving Communion, and as far as possible a Latin should follow suit.
Frequency in attending an Eastern celebration does not inscribe a Catholic to that rite, just as an Eastern Catholic who habitually attends the Latin rite does not automatically become Latin. To formally switch rites in a permanent manner requires a formal procedure.
The question is somewhat diverse for the case of Orthodox Churches, which are not in full communion with Rome but which enjoy the apostolic succession and all seven sacraments. While full communion is lacking, the Catholic Church no longer considers these Churches as being in a formal schism or as being excommunicated.
From the Catholic standpoint, a member of the faithful who is unable to attend Mass because there is no Catholic celebration available, may, if he so wishes, attend and receive Communion at an Orthodox Divine Liturgy.
Likewise, an Orthodox Christian in a similar situation is allowed to receive Communion and some other sacraments in any Catholic rite. Such an attendance is always optional and is never obligatory, not even in order to fulfill a festive precept.
However, not all Orthodox Churches accept this, and some take a dim view of any form of intercommunion. Once more it is incumbent upon Catholics not to impinge on others’ sensibilities and limit themselves to what is acceptable to each particular Church.
My only comment to this answer is that although many Orthodox Christians would be greatly offended if a Catholic were to receive communion at their church, there are places in the world (especially in the Middle East) where inter-communion between Orthodox and Eastern Catholic churches is commonplace.