Abstract: Epoxidation of an olefin is carried out by continuously reacting the olefin with hydrogen peroxide in the presence of a homogeneous epoxidation catalyst in a reaction mixture comprising an aqueous liquid phase and an organic liquid phase, using a loop reactor with mixing of the liquid phases. The loop reactor comprises a measuring section in which the liquid phases are temporarily separated, at least one pH electrode is arranged in the measuring section in contact with the separated aqueous phase, a pH of the separated aqueous phase is determined with the pH electrode and the pH is maintained in a predetermined range by adding acid or base to the loop reactor.