A range of esters including salicylates, nicotinates and parabens have been shown to undergo reversible, base-catalysed transesterification in hydroalcoholic solutions. In non-alcoholic solution phenyl salicylate undergoes a concentration-dependent oligomerization which yields salsalate and other products. The transesterification reactions also occur in products formulated for topical use, which have vehicles based upon alcohol, glycol or glycol polymers. Without recognition, such reactions may compromise stability assessments, pharmaceutical integrity and delivery profiles.