The flow microcalorimeter has been used for the determination of specific surfaces by heats of adsorption. The instrument was first calibrated by injecting known amounts of heat into plugs of different powders. It was found that small but significant differences occurred using different volumes of powder and different types of powder, although the effect due to different types of powder may well be a manifestation of the former effect since the packing arrangement (porosity) of the plug was not known. Langmuir theory and a mod fication assuming multilayer adsorption have been applied and it is shown that the latter gives closer agreement with nitrogen gas adsorption than the former and it is also shown that the conventional method of using a 0.20 v/v concentration of active solute is not satisfactory. In many cases it has been found necessary to clean the surfaces of solids by degassing before an analysis can be carried out. Tests have been carried out using a range of solvents, solutes and adsorbents.