Computer simulations of bubble oscillations in aqueous methanol solution are performed under conditions of sonoluminescence (SL) in order to study the mechanism of SL quenching by methanol reported by Ashokkumar [J. Phys. Chem. B 101, 10849 (1997); J. Phys. Chem. A 104, 8462 (2000)]. It is shown that methanol molecules evaporate into the bubble at the bubble expansion and almost all of them are dissociated inside a heated bubble at the collapse. The bubble temperature at the collapse decreases by the endothermal heat of the dissociation of methanol and by the increase of the molar heat of the gases inside a bubble, which results in the SL quenching. It is shown that SL from argon bubbles is more effectively quenched by methanol compared to SL from air bubbles.