In recent experiments the heat of chemical activation (formation of radicals) has been determined by measuring temperature coefficients of reaction rates of gases flowing through a furnace. To justify this procedure, an investigation is necessary to find out to what extent temperature equilibrium is reached. In this paper it is shown that the flow is laminar, and that two cases must be distinguished. Temperature equilibrium is reached throughout the flow when the pressure drop is less than 0.003 mm Hg per cm. For pressure drops about ten times as high, only a thin sheet close to the wall reaches temperature equilibrium, but this fact affects only the total number of activated molecules reaching the wall, not the temperature coefficients.