AbstractThe sterol 14α‐demethylation inhibitors 1‐(4‐substituted phenoxymethyl)‐2, 2‐dimethylpropyl imidazole‐1‐carboxylates and their corresponding thiocarboxylates were optimized for maximum in‐vitro activity against Botrytis cinerea Pers. ex Fr. in terms of the lipophilic parameter π. The activity of both carboxylates and thiocarboxylates was strongly related with π and predicted to be maximum at π 1.38 and 0.72 respectively. However, the preventive efficacy of the carboxylates against B. cinerea on Solanum melongena L. grown in a greenhouse did not correlate with the parameter. Despite the unfavourable lower lipophilicity, compounds with p‐alkoxy substituents were superior to those with any other substituent. Moreover, the p‐methoxy‐substituted thiocarboxylate 46 was much less effective in greenhouse tests than the corresponding carboxylate 15, despite their equivalent in‐vitro activity. To clarify these discrepancies, hydrolytic and photolytic stabilities of several representative compounds including 15 and 46 were investigated. The carboxylates examined were much more labile to hydrolysis than the thiocarboxylate 46; however, the four thiocarboxylates studied were less stable to light than the carboxylates. Consequently, the above discrepancies were attributable mainly to the superior stability of the carboxylates to photolysis as compa