Rotary kilns are commonly used for mixing of solids, such as grain, and heat transfer to solids, such as drying of fruit and calcination of cement. For a rotary kiln that has a wide range of solid feeds, it is desired to be able to know the kinetics of the mixing of solids inside the rotary kiln so that heat transfer between these solids can be predicted. Heat transfer between different solid feeds is particularly important in the rotating kiln being commissioned by Stuart Energy in Gladstone, Queensland for the processing of oil shale to produce oil. This paper describes a new way to determine the mixing rates of solids in a rotating drum using image analysis programming. Results of this analysis show that the mixing dynamics follow a constant rate until a completely mixed state is encountered. Upon closer analysis it was revealed that mixing occurred in steps, which has not been previously shown.