This paper examines y deletion in Seoul Korean on a large socio-linguistic database. It first shows that Seoul Korean has two distinct processes y deletion: categorical y deletion, which occurs after a palatal consonant, and variable y deletion occurring only before the vowel e. The first part of this paper examines variable y deletion. The examina- tion of the process reveals that it is conditioned by such internal factors as the existence of a preceding consonant, the nature of the preceding consonant, and word- internal position where y occurs, i.e., whether y appears in the word-initial or non- ward- initial syllable. This process is also affected by external factors such as the speech style, social class, and age of the speaker. The difference in the deletion rate of y among the three age groups (and among the social class groups, additionally) \nis taken to suggest that y deletion before e, i.e., monophthongization of ye to e, is a change in progress. This paper then attempts a phonological account of two different process of y deletion, proposing two OCP (coronal) constraints. The two constraints are proposed as having rather different strengths: the stronger one triggers categorical deletion and the weaker, variable deletion. A phonetic explanation of the ongoing y deletion change is also attempted. It is shown that ye has the shortest perceptual distance between its component segments among all y diphthongs of Seoul Korean. It is suggested that a relative lack of perceptual distinction between the two segments of the diphthong ye is responsible for the ongoing loss of y in this diphthongal sequence.