The purpose of this investigation was to identify the major parameters of significance in orthokinetic flocculation and to express their interrelationship in the form of a rate equation. Theoretical equations expressing the rate of flocculation of a polydispersed system were developed from the Smoluchowski equation and other considerations. The progress of flocculation in a series of continuous stirred-tank reactors was simulated by repeated computer application of the basic Smoluchowski equation and the computed floc size distributions compared with those predicted by the theoretically derived equations. The flocculation of kaolin by alum was experimentally studied in a laboratory-scale reactor and the results compared with those predicted by the integrated rate equation after substitution of parameters susceptible to laboratory measurement. (Modified author abstract)