Alcids dive longer than are predicted by their body size alone, but the physiological mechanisms that explain their excellent diving capabilities are poorly understood. In this study, we estimated the oxygen stores of Rhinoceros Auklets Cerorhinca monocerata, medium-sized alcids that attained depths down to 62 m within 2.5 min. Hematocrit was 43.9±2.8%, hemoglobin concentration was 17.2±4.6 g·100 ml^-1, and blood volume was 12.7±1.9% of their body mass. Myoglobin concentration in breast muscle (1.8±0.3 g·100 g^-1) was higher than that in leg muscle (1.2±0.2 g·100 g^-1). Rhinoceros Auklets have higher blood volume, hemoglobin and myoglobin concentrations, as do other flying/diving seabirds (other alcids and cormorants), than flying/non-diving seabirds (terns and kittiwakes). The oxygen store of Rhinoceros Auklets was estimated at 54.5 ml·kg^-1. Using the average oxygen consumption rate of diving seabirds (1.01 ml·s ^1·kg¹), we calculated their theoretical aerobic dive limit (TADL) as 53.9 s. Nearly half of their dives (47.2%) exceeded their TADL, because an overestimation of their oxygen consumption rate during diving resulted in an underestimation of TADL.