[FormerLib]

Nothing I've seen in the missalettes, and I do read them in Church, indicates that Catholics are allowed to take Communion in an Orthodox Church.

From EWTN:

It then addresses the question of Catholics receiving the sacraments from non-Catholics. It sets the following strict conditions:

a. necessity or genuine spiritual advantage
b. when the danger of error or indifferentism is avoided
c. it is physically or morally impossible to approach a Catholic minister
d. a church which has valid sacraments

This last condition is the key one, since it eliminates ALL the Reformation churches (Anglican, Episcopalian, Presbyterian, Methodist, Baptist etc.), none of whom have valid sacred orders, and therefore, a valid Eucharist. The possibility of a Catholic receiving from the minister of another church, when the first three conditions are fulfilled, is limited to the Orthodox Churches, other Oriental Churches, Old Catholics, Polish National and others whose sacraments are recognized by the Holy See.

[Destro]

I may be wrong but I do know that Orthodox can receive the sacraments from Catholics if no Orthodox clergy are present as a last resort - say like on a battlefield. If for example I was a soldier of the Orthodox faith and a Catholic clergy was the only alternative available on the battlefield I could accept the sacraments - participate in confession - the Holy Eucharist and the last rites, etc. Catholics and no others like Anglicans.