Methane and krypton adsorption measured on single-walled nanotubes at 78.7 and 77.3 K, respectively, give rise to stepwise isotherms, representative of the adsorption on two types of comparatively uniform patches during the first stages of the adsorbate condensation. The values of the methane isosteric heat of adsorption on these two fractions of quasi-uniform surface, determined from the dependence of the equilibrium pressure on temperature between 78 and 110 K, are in very good agreement with those measured at 77.3 K by means of isothermal microcalorimetry in quasi-equilibrium. Their respective mean values (18.3 +/- 1 and 11.2 +/- 0.5 kJ mol(-1)) enclose that of methane adsorption on graphite in the monolayer range (14.9 kJ mol(-1)). A phase transition occurring in the adsorbed film on the less attractive quasiuniform part of the surface can be predicted from volumetric measurements.