Intervocalic interchange between aleph and yod is most frequent in Hebrew. This article defines the conditions of this interchange in mishnaic Hebrew and presents a comparison with other Hebrew sources. The author demonstrates that it is necessary to distinguish the original yod from the original aleph. The original aleph always perdures, except when it serves as the third radical letter, in which case this should be read as a morphological rather than a phonological change (3rd radical aleph > 3rd radical yod). This evolution may be analysed along three parameters: 1) the phonological conditions: the interchange only occurs after a qamescombining dot below vowel, whereas after other vowels the yod persists throughout the entire rabbinical corpus; 2) the different branches of rabbinical Hebrew: the interchange occurs consistantly in the Babylonian, but not in the Palestinian branch; 3) rabbinical Hebrew vs other sources: the interchange is conditioned only in rabbinical literature, and not elsewhere. This demonstrates that rabbinical Hebrew constitutes an independant dialect with its own rules, differing not only from biblical Hebrew or from the Dead Sea manuscripts, but also from epigraphic sources of the rabbinical period.