Co-aggregation of fullerene C-60 and thiophene has been studied calorimetrically in cyclohexene at 25 degrees C. The total aggregation heat is found to depend on initial concentration of thiophene and fall between -1.9 and -5.8 kJ mol(-1). The corresponding thiophene/fullerene molar ratio ("co-aggregation number") ranges from 7 to 12. The data are rationalized by formation of heteromolecular nanoaggregates with intermolecular contacts of both fullerene-thiophene and fullerene-fullerene types. A physical model describing interaction between fullerenes and pi-donors in solution is substantiated and used to explain heterogeneity of composites containing fullerenes. (c) 2005 Elsevier B.V. All rights reserved.