In this work evidence is presented that acid stress adaptation protects Saccharomyces cerevisiae from acetic acid-mediated programmed cell death. Exponential-phase yeast cells, non-adapted or adapted to acid stress by 30 min incubation in rich medium set at pH 3.0 with HCl, have been exposed to increasing concentrations of acetic acid and time course changes of cell viability have been assessed. Adapted cells, in contrast to non-adapted cells, when exposed to 80 mM acetic acid for 200 min did not display loss of cell viability associated to morphological alterations typical of apoptosis. Thus, 80 mM acetic acid death-inducing conditions were selected to further characterize the early molecular events leading to such active cell death process. Catalase was specifically activated during acid stress adaptation and protection against acetic acid-induced death was associated with maintenance of its activity during treatment with 80 mM acetic acid. On the other hand, intracellular superoxide dismutase activity was found present at comparable levels both in adapted and in dying yeast cells, excepting in non-adapted cells which displayed a maximum activity value after 15 min acetic acid exposure, corresponding to more than 80% cell viability. This study gives first experimental evidence that H2O2, rather than superoxide, detoxification may have a major role in preventing yeast cell death in response to acetic acid. The results, as a whole, suggest that commitment of S. cerevisiae to a programmed cell death process in response to acetic acid is mediated through a ROS-dependent apoptotic pathway.