A calorimetric method was applied to the quantitative determination of the antimicrobial activity of Pionin-an additive used in the cosmetics industry-aspect previously investigated by other authors using more traditional methods. Because it is insoluble in the growth media, the additive required initial dissolution in 1, 3-butanediol or 1, 2-pentanediol-two ingredients widely used in cosmetics, which have been the subject of a previous research presented in this journal. Based on the changes observed in the heat evolution curves corresponding to microbial cultures to which Pionin was added in various amounts, quantitative parameters could be determined to characterize its antimicrobial action. Pionin exerted remarkable antimicrobial activity against Staphylococcus aureus and MRSA, was less inhibitory against Escherichia coli, Candida albicans and Aspergillus niger, and had reduced antimicrobial activity against Pseudomonas aeruginosa. With the exception of Candida albicans, Pionin was more efficient in inhibiting microbial growth when dissolved in 1, 2-pentanediol than when dissolved in 1, 3-butanediol.