The evolution of oxygen (O 2) intercalated in fullerene (C 60) solids has been studied by gas effusion spectra. The O 2 concentration in C 60 solids estimated from gas effusion spectra increases with exposure time and temperature. In the gas effusion spectra for samples with a high O 2 concentration, a new oxygen evolution peak around 200 °C is found in addition to the two evolution peaks around 80 °C and 150 °C observed in the spectra of samples with low O 2 concentration. In X-ray diffraction patterns of O 2-intercalated samples, a peak related to the intercalation of O 2 is found around the (111) reflection of the face-centred cubic structure. The lattice constant of samples with high O 2 concentration is expanded a little by annealing at 300 °C in vacuum. The intercalation and evolution of O 2 in C 60 solids are discussed.