The mixing performance of planar oscillatory flow reactors (planar-OFRs) with liquid solutions and solid suspensions was investigated. The planar-OFR is a relatively new variant of the OFR where the conventional sharp-edged periodic constrictions are replaced with smooth periodic constrictions and the conventional circular cross-section is replaced with a rectangular cross-section. The experimental results show that both the minimum mixing intensity required to fully suspend solids and the time required to reach a specific degree of concentration uniformity after injection of a liquid or solid tracer largely depend on the operating conditions, such as the frequency and amplitude of oscillation and the geometric parameters, such as the cell aspect ratio and the ratio of the open constriction area to the straight section area, in the planar-OFR.