In Modern Maltese obstruents in clusters agree in voicing. Earlier Maltese, however, did not have a rule of voicing assimilation. The present paper reconstructs the diachrony of voicing assimilation on the basis of the spelling used in records of earlier Maltese. It is shown that voicing assimilation appears to have become a rule of Maltese towards the end of the 18th century. Diachronically, voicing assimilation is therefore a case of rule addition and illustrative of lexical diffusion. The combined effect of the rules of voicing assimilation in obstruent clusters and of word-final obstruent devoicing is a typological shift.