A simple description of a new flow calorimeter constructed in our laboratory for the measurement of multicomponent vapor heat capacity is given. The vapor heat capacities of benzene, a standard substance, at the temperature range 375 less than or equal to T/K less than or equal to 575 at atmospheric pressure, were determined with this apparatus, the uncertainty of which was less than 0.7 per cent. The vapor heat capacities at various temperatures but at p = 101.3 kPa of(acetic acid + 4-methyl-2-pentanone) were determined with this calorimeter. The effect of association in the vapor phase on the vapor heat capacity was discussed. The results showed that there is strong association among acid molecules in the vapor phase and that vapor heat capacities of the mixture are negative compared with those of ideal mixtures.