The utility of the commonly applied isotherm equations in a wide range of temperatures and pressures was studied using the sorption data of supercritical hydrogen on activated carbon. The equilibrium condition covers 77-298 K and 0-7 MPa. The isotherm models tested include the Langmuir equation, the generalized Freundlich equation, the virial equation, and the Dubinin-Astakhov equation. The fitness is quantitatively expressed by standard deviations at each temperature and general deviation for the whole range. Discussions and inferences are presented on the applicability of the equations and on the state of the adsorbate at supercritical temperatures.