A method has been developed for measurement in vitro of the simultaneous activities alcohol acetyltransferase (AATase) and ester hydrolase (EHase) in yeast extracts taking into account the possibility of interaction between the two opposite activities or the rapid inactivation of the AATase activity. A mathematical model. including as parameters a first order kinetic constant corresponding to the EHase activity and the inactivation constant of AATase. is proposed for the evaluation of AATase activity. To determine ester concentrations, the Headspace-SPME-GC technique has been used. The method has been successfully applied to three yeast strains belonging to the species Pichia anomala, Pichia heedii and Saccharomyces cerevisiae and the corresponding parameters for AATase and EHase have been calculated.