The temperature and hydrogen pressure dependences in the ring-opening reactions of methylcyclobutane were studied over Pt/SiO2 catalyst. The temperature dependence of the ring opening revealed that the reaction rate vs. temperature curves passed through a maximum. On the basis of this information two temperatures were selected for hydrogen pressure studies : 573 K, close to the lowest temperature at which any reaction took place at all, and 623 K, where the ring opening of methylcyclobutane exhibited the highest rate. The initial formation rate vs. hydrogen pressure dependence curves are of bimodal type at 573 K, but they increase monotonously at 623 K. Over the working catalyst, no significant changes were observed at 573 K, but the curve for pentane formation changed to a large extent at 623 K. At 573 K, the selectivity of ring opening was close to statistical, with little excess of isopentane (sterically less hindered direction) over both the clean and the working catalyst. This was also observed at 623 K, however, over the working catalyst as the hydrogen pressure increased the selectivity of the ring opening increased as well. Moreover, at the highest hydrogen pressures studied excellent selectivity for the formation of isopentane was observed. The mechanisms over the initial and the working catalysts are discussed on the basis of these experimental findings.