Old Persian shows a change of postconsonantal y, w to iy, uw, respectively. However, applying (pre )Middle Persian sound changes to the Old Persian forms, the result is at variance with certain Middle Persian forms. If one were to assume a syncope reversing the Old Persian change of y, w to iy, uw, this would affect also old cases of iy, uw and likewise yields wrong results in Middle Persian. The Old Persian change can thus not have operated in the prehistory of Middle Persian; there is thus a dialectal difference between attested Old Persian and the later stages of the language, which is to be added to those already noted. I also discuss some sound changes that chronologically depend on the Old Persian change in one way or the other, thus collecting evidence for the relative chronology of Persian historical phonology. The appendix presents a tentative sequence of the processes discussed in this article.