A simple calorimeter has been devised by means of which the heats of mixing (molar excess heats) of the following systems have been determined at 22 °C: acetone – acetic anhydride; acetone – carbon disulfide; acetic anhydride – carbon disulfide; and a pseudo-binary section of the ternary system acetone – acetic anhydride – carbon disulfide. From these data the partial molar excess heats have been obtained. All systems are very non-ideal except the system acetone – acetic anhydride, which is almost ideal.For the partially miscible system acetic anhydride – carbon disulfide, the excess thermodynamic quantities of mixing, GE, HE, and TSE, have been calculated,