Abstract Chong (2017) claims that a derived environment process is productive to the extent that it is supported by phonotactics. The present study tests this claim by comparing variable patterns observed in Korean vowel harmony of suffix alternation with vowel co-occurrence restrictions in the lexicon. A corpus study replicated findings of previous studies that the harmony in suffix alternation is losing productivity, conditioned by the quality of the stem vowel, the number of intervening consonants between the vowels and the stem class. Phonotactic generalisations in vowel sequences matched such tendencies in the alternation in that harmony was feeble in phonotactics and that some of the factors that modulate the harmony in alternation were found to affect the harmony in phonotactics as well. The findings generally support Chong’s claim that lack of phonotactic support for an alternation makes it harder to learn.