A mathematical model of compressive thickening is developed which is based on Kos’ model yet can be calibrated from measurements of concentration, concentration gradient, and bulk underflow velocity in continuous thickening experiments. The need for sensitivity and difficult measurements such as interparticle pressure or volumetric solids concentration is eliminated. Numerical programs to simulate the model are described. The model was calibrated and tested using steady state continuous thickening experiments. Unsteady state continuous thickening was studied by use of computer simulation of the model. Programs are described for nonlinear regression of the models to continuous thickening column data, and for steady state and unsteady state simulation.