A nationwide network of regionally located Standard Reference Photometers (SRP) for the assay of ozone concentrations has been established to allow state and local air monitoring agencies to compare their ozone standards with authoritative standards maintained and operated under closely controlled conditions. The SRP was developed by the National Bureau of Standards and the Environmental Protection Agency (EPA) as a highly stable, highly precise, computer-controlled instrument for the assay of ozone concentration. EPA's Environmental Monitoring System Laboratory at Research Triangle Park, NC is operating an SRP network in cooperation with EPA Regional Office or State Agencies. Currently, network sites are located in RTP, NC; Edison, NJ; Chicago, IL; Houston, TX; Denver, CO; and Sacramento, CA. Each network SRP was fabricated and certified by the NBS before deployment and is recertified annually by EPA. To date 86 comparisons of local 03 standards have been performed with the network SRP's. Of the 55 verifications of local 03 primary standards conducted, 46 comparison results (84%) were within the acceptable range (+ or - 3% agreement). Of the 31 verifications of local 03 transfer standards conducted, all 31 comparison results (100%) were within the acceptable range (+ or - 5% agreement) with 29 (94%) within + or - 3%.