The mechanistic aspects of the one-pot C-acylation of cyclic 1,3-diones are discussed. Using various substituted carboxylic acids, coupling agents and basic catalysts we aim to describe the process of direct transfer of an acyl group to the activated C-2 carbon of dimedone and 1,3-cyclohexanedione. The course of acylation was studied by high-performance liquid chromatography as well as on-line infrared spectroscopy. The results indicate that the direct acylation follows a nucleophilic acyl substitution mechanism rather than a nucleophilic attack on a ketene intermediate formed in situ from the activated carboxylic acid.