This study traces the development of the adversative and neutral passives in Thai to draw inferences about their establishment and change. It shows how the word /thùuk/, which used to occur with only a verb with unfavorable meaning to form an adversative passive, is now used with virtually all transitive verbs in Thai. This suggests that the neutral passive has been established in Thai syntax. However, /thùuk/ is ambivalent because it marks both adversative and neutral passives. This means that the adversative passive still persists in Thai, but it is now likely to be marked by /doon/, which has a clear adversative meaning. This passive marker seems to have developed in the track of /thùuk/ and is replacing the equivocal /thùuk/ marker.