Key measurements in the National Gallery of Art Research Project's program of evaluation of thermoplastic resins are reviewed. New data on photochemicaliy induced loss-in-weight of polymers is reported and evidence of chain breaking noted in two highly stable polymers that are resistant to cross-linking. Also reported are preliminary data on the increase in solvent power necessary to remove varnish resins as a function of the time of exposure to ultraviolet and visible radiation. To provide standards of photochemical stability, three classes of lightfastness are proposed.