Flow visualization studies were carried out to characterize the particle mixing behavior under rotational conditions employed in previously reported heat transfer studies but at room temperature. Containers were subjected to end-over-end rotation in the rotary retort, with the door held in an open position for video taping of particle mixing in the container Particle concentration (20, 30 and 40%, v/v), particle size (diameter of spherical particle 0.019, 0.022 and 0.025 m), particle shape (sphere, cylinder and cube), liquid viscosity (1 x 10(-6) and 1 x 10(-4) m(2)/s) and rotational speed (10, 15 and 20 rpm) were studied as variables. Analysis of variance on heat transfer data obtained from previous studies showed that the effects of all five factors studied on overall heat transfer coefficients were significant (p < 0.05). These factors affected fluid to particle heat transfer coefficients even more significantly (p < 0.0001). The flow visualization study revealed that the differences in the heat transfer coefficients could be explained by the particle mixing behavior.