Equilibrium constants for the Diels-Alder reaction of 9-chloroanthracene with tetracyanoethylene have been determined in the pressure range 0-970 kg cm(-2) in 12-dichloroethane at 298 K and the value of the reaction volume (-20.6+/-1.5 cm(3) mol(-1)) has been compared with the difference in partial molar volume of the dienophile (107.8+/-0.2), diene (170.7+/-0.5) and adduct (255.5+/-1.5 cm(3) mol(-1)).