Extensive improvements have characterized the evaluation of the GRC Photochemical/Diffusion model. Despite the limitations of smog chamber experimental data, they have served an essential purpose toward updating the kinetics portion of the model. Consistency of rates and reactivities is now achievable using recently measured coefficients for a wide variety of systems. Model methodology revisions have enhanced the realism of the advective and diffusive descriptions. Previous assumptions regarding transverse (cross-streamline) horizontal diffusion have been confirmed by an exhaustive series of parametric tests. Photochemical/diffusion validations were successful for trajectories occurring during four days of the 1969 smog season in Los Angeles. The measure of success is seen in concentration-history fidelity with a minimum of adjustments of diffusion parameters. (Chemical coefficients were scaled from the smog chamber studies and held fixed for the simulations carried out to date). Future directions for air pollution model development are discussed in detail in an appendix as information supporting the experimental recommendations. (Author)