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Resulting from a specific quest among the archives of the religious institutions in Paris, this article first figures out the correct numbers of Jews in Paris who, from 1807 to 1914, forswore their own faith and converted to Catholicism, as well as the number of Christians who converted to Judaïsm during the same period. From this research we can make out that while 1 000 Jews became Catholics, nearly 500 Catholics turned to the Jewish religion. Then Judaïsm did not such a dramatic drain because of conversion, which comes in fact as a minor reason in the losses of Judaïsm. The accurate analysis of the ebb and flow of conversions according to periodes, sex, age and social classes goes together with a study of the motives among which mariage comes widely ahead, especially when dealing with conversion to Judaïsm.