For the two binary liquid systems {xC(6)H(5)CN + (1 - x)C6H6} and {xC(6)H(5)CN + (1 - x) C6H5CH3}, excess molar volumes V-m(E), excess molar enthalpies H-m(E), and excess molar heat capacities C-p, m(E) at constant pressure have been measured as a function of mole fraction x. Values of V-m(E) were determined at T = (298.15, 308.15, and 318.15) K, H-m(E) was determined at T = (293.15, 298.15, and 303.15) K, and C-p, m(E) was determined at T = 298.15 K (all at atmospheric pressure). The instruments used were, respectively, a vibrating-tube densimeter (from Sodev), an LKB differential flow microcalorimeter equipped with two computer-controlled h.p.l.c. piston pumps (from Gilson), and a Picker flow microcalorimeter (from Setaram). In addition, isobaric heat capacities divided by volume C-p,C- m/V-m of the pure liquids, as well as several selected mixtures, were measured with a programmable differential scanning calorimeter of the Calvet type (micro-d.s.c., from Setaram) between approximately T = 280 K and T = 350 K. Both systems show relatively small negative excess molar volumes, which become more negative with increasing temperature. The excess molar enthalpies are highly unusual in that for both systems an M-shaped composition dependence is observed (two maxima and one minimum). The M-shape is much more prenounced for(benzonitrile + toluene) than for (benzonitrile + benzene), and appears to vanish for the latter system below T = 298.15 K. The results can be understood in terms of a simple theory of complex formation (Guggenheim-McGlashan). The excess molar heat capacity at constant pressure at T = 29815 K of (benzonitrile + toluene) is positive at all compositions, while that of (benzonitrile + benzene) is positive only for 0 < x < 0.071, and negative otherwise (sigmoidal shape). Combining the heat capacities obtained with the Picker calorimeter at T = 298.15 K with our results for C-p,C- m/V-m obtained with the micro-d.s.c. in conjunction with our density data, excess molar heat capacities have also been derived for (benzonitrile + toluene) at T = (288.15, 308.15, 318.15, and 328.15) K. For this system, the maximum of the curve C-p, m(E) against x becomes somewhat smaller with increasing temperature and is shifted towards larger values of x, i.e. the curves become more symmetric.