The synthesis of propylene oxide from propylene and hydrogen peroxide and the side reactions of propylene oxide were studied in a broad range of experimental conditions (25-80 degrees C, 2.5-8.5 bar) in a laboratory-scale trickle-bed reactor using commercial titanium silicate (TS-1) as a heterogeneous catalyst. The reaction solvent was methanol. A very precise gas chromatographic analysis method was developed to study the reactant conversion and product distribution. Long-term experiments in the continuous reactor revealed excellent catalyst stability. Besides the main product propylene oxide, side products such as 1-methoxy-2-propanol and propylene glycol were formed via ring-opening reactions with methanol and water. A prominent maximum in the activity at optimal temperatures was observed. The formation of the side products and the catalytic activity was related to the concentration of water and hydrogen peroxide. An extensive study of propylene oxide transformations was conducted, illustrating the importance of lower water concentration and shorter residence times for selective production of propylene oxide. The reaction mechanisms of the epoxidation process and the side reactions were discussed: a consecutive-parallel reaction scheme was confirmed by the propylene synthesis experiments and separate experiments with propylene oxide and methanol and water.