A new flow calorimeter has been constructed to determine the isobaric heat capacity of vapors, especially multicomponent vapors. The calorimeter is capable of operation up to 673 K acid 20 MPa. A novel design encloses the innermost part of the calorimeter within a double set of thick-wall, aluminum shields, replacing the traditional constant temperature bath, to minimize heat loss and precisely control the calorimeter temperature. An upper copper block within the outer shield is used to preheat the fluid while a lower copper block within the inner shield further heats the fluid to the operating temperature. Such a structure and temperature-controlling arrangement allow easy component replacement and operation. The apparatus was tested by determining the heat capacity of benzene and the benzene-carbon tetrachloride binary system at atmospheric pressure with an experimental error of less than 0.7%. Results for the two binary systems benzene-chloroform and benzene-acetone at 101.3 kPa and about 515.0 K are also given.