The influence of varying the paste-to-paste ratio of chemically initiated composites on the working time, diametral tensile strength and in vitro 'occlusal' wear was investigated. If the mixing ratio was varied, the working time was not equally affected for the various brands. This is explained by different relative inhibitor concentrations in the pastes. Within reasonable deviations from the 1:1 ratio, no significant influence on the diametral tensile strength occurred. Wear properties as determined by a simulated occlusal wear test on three posterior composites, was more sensitive to the mixing ratio.