The effects on cognitive performance of breathing air, oxygen and nitrox gas mixtures at surface ambient pressures were investigated during an expedition to the Everest region of Nepal. A slight improvement in grammatical reasoning at altitude was found under nitrox (p<0.05) and mathematical reasoning showed improvement at altitude on air (p<0.05), oxygen (p<0.01) and nitrox (p<0.01). There were non-significant trends towards decreasing mathematical ability, coupled with an increase in variance on both grammatical and mathematical test performance, with increasing pO2 (all p>0.05). The results suggest that there is a subtle interaction on cognition as indicated by a significant three-way interaction between subject x altitude x gas (p<0.05).