After a rather complete exploratory program described in previous papers, the photothermoelastic method was applied to the experimental evaluation of thermal-stress theories. The new technique was correlated with several theories which analyzed the transient thermal stresses in idealized wing structures of high-speed aircraft. Various theories were investigated which represented the same idealized wing models and differed from each other only in the simplifying assumptions regarding the temperature distributions in skin and webs. The theories were evaluated by duplicating the boundary and initial conditions on plastic models and then by correlating the theories with the observed fringe orders in nondimensional form. A significant general conclusion was reached after correlating the available theories and experimental results. Owing to simplifying assumptions concerning the thermal behavior in the flanges, thermal stresses predicted by the available theories are all higher than the experimental observation. In some cases the discrepancy is as great as 30 per cent.