The mixing between transient puffs of evaporating waste and main burner flue gas in a rotary kiln incinerator was studied through cold flow model experiments and computer simulations. The experiments consisted of the temporary release of a particle laden stream into a cross flow, and the subsequent visualization of the concentration field using laser sheets or laser lines. Quantitative data, including average concentration and concentration fluctuation profiles, were obtained from video frames by digital image analysis. The experiments were compared to a computer simulation model, based on the linear eddy modeling technique. Model results agreed very well with experimental data, if a recirculation zone was explicitly accounted for. This study suggests that gas phase unmixedness is a likely explanation for failure modes in rotary kiln incineration. Insufficient macro-mixing (controlled by transient evaporation and large scale motion) is more important than local unmixedness (governed by fine scale turbulence and molecular diffusion), although both processes play a role. Linear eddy model simulations also showed that low resolution measurements of mixing with large Schmidt numbers (i.e. for particles) can be interpreted as fully resolved mixing with Schmidt number equal to unity (i.e. for gases).