Pope Benedict has lifted restrictions on celebrating the Latin Tridentine Mass, pleasing some traditionalists.
Some priests already celebrate the traditional Mass
The Latin Mass was largely abandoned in the 1960s, as part of reforms to make Catholicism more relevant to its worldwide congregation.
Traditionalists wanted to bring the Mass back, though some Jewish groups opposed it because of a prayer calling for their conversion.
The Pope denied claims the reversal could cause a schism in the Church.
The late Pope John Paul II partially relaxed the prohibition in the 1980s, allowing bishops discretionary powers to let priests celebrate Mass in Latin if members of the congregation asked for it.
The Pope wanted to heal a rift with ultra-traditionalists who rebelled against Second Vatican Council changes.
The Church believes the majority of its congregation will continue to hear Mass in their local languages.
Catholic commentator John L Allen told the BBC in April he did not believe there would be much call for the Mass - and 40 years after the Second Vatican Council, there would be few priests able to read it.