Male athletes tend to outperform female athletes in several endurance sports. Maximum cardiac output can be estimated by maximal oxygen consumption (V˙O2max), and it has been established that men presentV˙O2maxvalues about 20% higher than women. Although sex differences inV˙O2maxhave already been well studied, few studies have assessed sex differences with regard to muscle oxidative capacity. The aim of this study was to compare aerobic muscle quality, accessed byV˙O2maxand adjusted by lower limb lean mass, between male and female amateur triathletes. The study also aimed to compare sex differences according toV˙O2submaximal values assessed at ventilatory thresholds. A total of 57 participants (23 women and 34 men), who had been training for Olympic-distance triathlon races, underwent body composition evaluation by dual-energy X-ray absorptiometry and performed a cardiorespiratory maximal test on a treadmill. Male athletes had significantly higherV˙O2max, both absolutely and when adjusted to body mass. Conversely, whenV˙O2maxwas adjusted for lean mass, there was no significant difference between sexes. The same was observed at submaximal exercise intensities. In conclusion, differences inV˙O2maxadjusted to body mass but not lean mass may explain, at least in part, sex differences in performance in triathlons, marathons, cycling, and other endurance sports.