IL-33 activates eosinophils directly via the ST2 receptor. Like IL-5, IL-33 induces eosinophilia and eosinophilic airway inflammation in mouse models and primes human eosinophil responses. Previously, we reported that IL-5 priming enhances Siglec-8 mediated mitochondrial and reactive oxygen species (ROS)-dependent eosinophilic apoptosis and eliminates caspase dependence of this cell death process. Whether IL-33, like IL-5, augments pro-apoptotic pathways involving receptors such as Siglec-8 and in a similar manner has not been explored. Annexin-V labeling was performed to detect apoptosis in human eosinophils pre-incubated with or without a range of concentrations of IL-33 and/or IL-5 in the presence or absence of Siglec-8 monoclonal antibody (mAb) 2C4 and inhibitors of caspases. Tetramethyl-rhodamine staining was used as a marker of mitochondrial membrane potential loss and injury. ROS production was determined by measuring the superoxide dismutase-inhibitable reduction of cytochrome c. Cleavage of poly(ADP-ribose) polymerase (PARP) was assessed using Western blotting. Eosinophils cultured alone or with mAb 2C4 underwent low levels of apoptosis at 24h. 2C4-induced eosinophil apoptosis was markedly and equally enhanced after culture for 24h with either IL-33 or IL-5, although IL-5 was more potent. Effects on apoptosis with IL-33 and IL-5 were synergistic. In contrast, percentages of cells exhibiting reduced mitochondrial membrane potential were greater with IL-33 than IL-5 and effects of these cytokines were also synergistic. Antimycin, an inhibitor of mitochondrial electron transport, almost completely inhibited 2C4-induced apoptosis with either IL-33 or IL-5. Surprisingly, 2C4-induced eosinophil ROS production was significantly enhanced with IL-5 but not IL-33. Siglec-8-mediated apoptosis in the presence of IL-33 was more sensitive in magnitude than IL-5 to inhibition by the pan-caspase inhibitor Z-VAD-FMK, yet both cytokine conditions were associated with PARP cleavage. These data demonstrate that IL-33 is as effective but less potent than IL-5 in enhancing Siglec-8-mediated eosinophil apoptosis, and can synergize with IL-5. Eosinophils primed by IL-33 and/or IL-5 in vivo would be expected to display enhanced susceptibility to undergoing Siglec-8-induced apoptosis.