It is commonly stated that mixtures of 80% helium and 20% oxygen (helox) increase thermal conductance, and hence heat flux, from homeotherms by roughly a factor of two. However, because helox affects heat loss by conduction and convection differently, its effect on heat flux should vary according to the relative contributions of these two routes to the overall heat flux. We used heated models made of three sizes of copper tubing and covered with various grades of synthetic fur to measure heat flux under air and helox atmospheres. Thermal conductance in helox (C_helox; W•m⁻² •°C⁻¹) was highly correlated with thermal conductance in air (C_air) and could be predicted by C_helox=1.704+1.726 C_air. The relative increase in heat flux under a helox atmosphere compared with that in air (conductance ratio; C_helox/C_air) varied from a maximum of 2.5 when thermal conductance is infinitely small and heat flux is predominantly determined by conduction to a minimum of 1.85 when animals are naked and heat flux is dominated by convection. These values follow the relation: C_helox/C_air=2.503−0.081 C_air. Body size has no significant effect on the conductance ratio.