trans,trans Isomers of sorbic acid, its potassium salt and ethyl sorbate isomerise under UV irradiation. All four geometrical isomers of the acid, salt and ester were separated using high-performance liquid chromatography on a nonpolar reversed-phase column (C18) and the ester also by gas chromatography on a VOCOL capillary column. The limit of detection and the interval of linearity were determined for all chromatographic methods. Individual isomers were identified with NMR analysis. Resolved chemical shifts of protons adjacent to the double bonds enabled qualitative and quantitative determination of isomers in the mixture. Antimicrobial activity of potassium sorbate isomers was tested on yeast Saccharomyces cerevisiae. Results show that the pure trans,trans isomer has a higher antimicrobial activity than the mixture of isomers.