The rearrangements between the closed [6,6] and open [5,6] isomers of C60O, as well as the closed [6,6], closed [5,6], and open [5,6] isomers of C60S have been studied using semiempirical AM1 and MNDO methods. The results show that the interconversion of the two isomers of C60O follows a two-step pathway involving an intermediate and two transition states. The calculated activation barriers for the migration of oxygen from [6,6]-bond to [5,6]-bond through an intermediate are 189.1 and 54.6 kJ mol(-1), respectively. In the opposite way, the calculated activation barriers for the migration of oxygen from [5,6]-bond to [6,6]-bond through an intermediate are 293.2 and 2.7 kJ mol(-1), respectively. The interconversion of the closed [6,6] and the open [5,6] isomers of C60S also follows a stepwise pathway via a local energy minimum corresponding to the closed [5,6] isomer. The calculated activation barriers for the migration of sulfur from [6,6]-bond to [5,6]bond (in open [5,6] isomer) through the closed [5,6] isomer are 233.2 and 1.2 kJ mol(-1), respectively. In the opposite way, the calculated activation barriers for the migration of sulfur from [5,6]-bond (in open [5,6] isomer) to [6,6]-bond via the closed [5,6] isomer are 82.0 and 150.5 kJ mol(-1), respectively. The large barriers suggested that it should be possible to isolate the closed [6,6] and open [5,6] isomers of C60O at room temperature. This is consistent with experimental results. In addition, it can be inferred from our results that rearrangement between the two isomers of C60O can take place under heating or lighting conditions. Meanwhile, it can be deduced from our results that it should be possible to isolate both the closed [6,6] and the open [5,6] isomers of C60S at room temperature and to convert one into the other when certain energy is offered. It seems that the closed [5,6] isomer of C60S may not be observed experimentally at room temperature.