Oil shale retort waters contain a large variety of substances which singly or in combination serve to make them very difficult to treat. The subject research evaluates the treatability of four oil shale retort waters using various physical chemical unit processes. The main emphasis in this effort is placed upon evaluating the efficacy of coagulation-flocculation. Low dosage levels for removal of true suspended solids are determined for each of the retort waters. Treatment efficiencies and optimum process variables are developed and compared.