Antifungal activity of four weak organic acids against S. cerevisiae cells were examined. Antifungal effects of hexanoic (C6), octanoic (C8), decanoic (C10) and benzoic acids were determined through Minimum Inhibitory Concentration (MIC), and inhibition zone measurements. The most effective weak acid was decanoic acid (MIC: 0.2-0.3 mM) according to MIC results. In order to have some insight in the inhibition mechanism of these weak acids, their efficiency was compared with that of hydrochloric acid (HCl) giving the same amount of drop in extracellular pH. Results demonstrated that the inhibition of yeast cells by weak acids is not simply due to acidity, but toxic effect of the anion and the insertion of the weak acids inside the cellular membrane may play a role. Moreover, synergistic effects of weak acids were examined and it has been shown that combinations of weak acids are more effective than using weak acids alone. Thus, this research not only opens new perspectives on antifungal activity mechanisms of weak acids, but also help their usage in combination widely.