The Vatican says it will open its secret archives from 1922 to 1939 with priority given to relations between the Holy See and Germany. So far, only documents up to 1922 have been made available to researchers.
The decision to open those from the papacy of Pius XI (1922-39) ahead of schedule was taken by Pope John Paul II.
The move is seen as a reaction to complaints of Vatican secrecy in its relations with Nazi Germany and its role during the Holocaust.
Before becoming pope, Pius XII was Vatican ambassador to Nazi Germany and afterwards secretary of state.
In 2000, the Vatican, which was pushing for the beatification of Pius XII (1939-1958), formed a special commission of three Catholic and three Jewish scholars to examine documents, selected by the Vatican, from 1939-45 in a bid to evaluate the role of Pius XII during the second world war and the Holocaust.
In July 2001, the commission abandoned the job in protest at not being given access to the rest of the archives.