The radioactive tracer technique was used to evaluate the liquid and solid (fine and coarse) transport in two industrial semi-autogenous grinding (SAG) mills. The residence time distribution (RTD), along with the mean residence times, was obtained. The residence times for the liquid and fine solid were equivalent, approximately 2 min, which implies non-significant segregation in this size class. The residence time for coarse solids was approximately 4 min. The RTD was modelled by N perfectly mixed tanks in series, in which the number of equivalent mixers was approximately 2 for liquid and fine solids and approximately 3 for the coarse solids. The coarse particles required additional time to pass the grate discharge restriction, which also favoured the mixing.