Microwave heating is not the same as conventional heating, and it is believed that this difference, the "microwave effect," may be interpreted to be due to selective, local heating. The temperature at the surface where sorption occurs is "effectively" greater than the measured solid or gas temperature. In these studies, measurements of the amounts of adsorption as functions of the partial pressures of a specific adsorbate in the presence of microwave irradiation were related to the conventional adsorption isotherms. Equating the adsorbate pressure required to achieve a specific coverage (an isostere) in the presence of microwave irradiation to the amount adsorbed for a conventional isotherm allowed for an estimate of the "effective" surface temperature in the presence of microwaves. It was found that the effective surface temperature increased when using adsorbates having a higher permittivity or when increasing the microwave power. The implication of this change in the surface energy for specific species in the presence of microwaves is discussed.