Most reactions involving a polymeric reactant are, or can easily become, diffusion‐controlled. While phenomenological models have been developed, direct fundamental knowledge of the diffusion, and data on the diffusion coefficients are lacking. Data on small molecule and polymer self‐diffusion coefficients are being obtained by two optical techniques over wide ranges of polymer concentration (0–100%) and diffusivity (10−14 to 10−5 cm2/sec). We present here a summary of our work of the last year in diffusion measurement by two different techniques. Forced Rayleigh Scattering (FRS) is being used to study small molecule diffusion in polymer fluids. Dynamic light scattering is being used to study polymer molecule diffusion over same range of variables. We hope through this work to be able to provide a complete picture of molecular mobility under polymerization conditions.