A method for the determination of mixing times was developed and tested in a model Nauta mixer filled with sand.128I-labelled sand particles were used as radiotracer and the progress of the mixing process was monitored with two externally situated radiation detectors. The variance of the radiation intensity decreases with time until a constant value is reached indicating an equilibrium mixture of tracer and sand. The time necessary to achieve this condition (mixing time) appears to be dependent on the rotational speed of the hopper as well as the screw of the mixer. Optimal mixing performance (shortest mixing time) corresponds to the maximal rotational speeds of both rotating parts of the mixer.