Measurements of heats of adsorption of ammonia from a nitrogen carrier have been carried out on zeolite Y and K10 clay catalysts in the acid and Na+ forms. The equipment used was a new model of the Microscal flow calorimeter (FMC) linked to a thermal conductivity detector in which the rates of adsorption and desorption and the associated rates of heat evolution or absorption were measured simultaneously at atmospheric pressure and temperatures ranging from 150 to 207 degrees C. Ammonia was used as a probe interacting with the acid sites on the catalysts. The work revealed new information in surface heterogeneity of the catalysts existing under flow conditions at atmospheric pressure of a nitrogen carrier. It was found that the irreversibly adsorbed ammonia is mobile on both catalysts. Molar heats of adsorption recorded as surface coverage is increased show that the sites covered first are not necessarily those with the highest heats of adsorption, illustrating that this flow technique gives important information on the relative accessibilities of acid sites as well as their strengths.