C60 fullerene and pentacene were reacted under mild conditions in toluene, yielding the monoadduct, which was characterized by electronic absorption and Fourier transform infrared (FT-IR) spectroscopy. The C60/pentacene adduct was studied by thermogravimetric analysis and its experimental composition was found in line with the theory for a 1:1 molar adduct. The decomposition temperature of the C60/pentacene monoadduct was found to be 404°C. The decomposition implies a retro Diels–Alder reaction, yielding back C60, which was recognized by FT-IR spectroscopy, while pentacene underwent decomposition and vaporization at 404°C. When an equimolar mixture of C60 and pentacene was heated in a differential scanning calorimeter in absence of solvents, the formation of monoadduct occurred at 380°C, but above 400°C it was decomposed back to reactants.