Experimental investigation on RTD of solids is carried out in a single-stage fluidized bed provided with an internal, using uniformly sized particles and a,binary solid-mixture, varying gas flow rate, solids rate, bed height, dilution and the bed,geometry. The effect of these variables on first and second moments as well as on F-curves, has been determined. Using a binary solid mixture or an internal inside the bed is found to reduce backmixing of solids. The data is fitted to FTEM and the values of N obtained were compared for different variables.