The aim of the study was to correlate diffusivity of Niacinamide from a cream formulation and Diclofenac sodium from a gel formulation. Evaluation is done by in vitro drug release through Franz diffusion cell and different diffusion membranes. The membranes used for the study were Polytetra fluoroethylene (PTFE), Polyvinylidene fluoride (PVDF) and Dialysis mem branes. Phosphate buffer pH 5.5 was the receptor medium. Studies showed significant variations in the diffusion rate of niacinamide and Diclofenac sodium across different membranes. The percentage of Niacinamide diffused from niacinamide cream after 120 min was found to be PTFE (1.5%), PVDF (54.15%) and Dialysis membranes (21.6%). The percentage of diclofenac sodium diffused from diclofenac gel after 180 min was found to be PTFE (2.42%), PVDF (14.42%) and Dialysis membranes (9.8%). From the study it was concluded that membranes having different characteristic property shows different diffusivity. Therefore interpretation of in vitro diffusion data to correlate with in vivo dermal penetration need further indepth study.