The enthalpy of solution of C-60 in toluene, carbon disulfide, decalin, and N,N-diethylaniline and of C-70 in CS2 has been measured using an isoperibol solution calorimeter. Exothermic heats of solution are observed in all cases. The heat of solution of C-60 in toluene is -8.6 +/- 0.7 kT mol(-1); in carbon disulfide it is -20 +/- 1 kT mol(-1). In decalin, slow dissolution prevents an accurate determination, but the heat of solution is less than -5 kJ mol(-1). In N,N-diethylaniline, a fast exothermic step comparable to that observed in other systems is followed by a much larger heat evolution, implying that a chemical reaction is occurring. The heat of solution of C-70 in CS2 is -9 +/- 1 kT mol(-1). A thermodynamic theory of the temperature dependence of fullerene solubility is presented. The central idea is that the unusual temperature dependence of the solubilities of fullerenes is caused by the formation of solvated crystals. Conditions are derived under which a temperature maximum of solubility, caused by an incongruent melting point of a solvated crystal, will occur. The calorimetric results suggest that the negative temperature dependence of the solubility of fullerenes may be quite general if the solid phase in equilibrium with the saturated solution is unsolvated fullerene.