The color change resulting from storage in water was measured on a number of experimental and proprietary composite resins. The experimental resins contained varying amounts of amine (DEPT or DEBA), benzoyl-peroxide (BPO) and inhibitor (MHQ). The proprietary resins had earlier been analyzed as regards the composition of the monomer and the content of amine and peroxide. The color change of DEPT-containing resins was positively correlated with the DEPT/BPO ratio, and negatively correlated with the monomer content of BISGMA. DEBA-containing resins were more color stable than resins containing equimolar concentrations of DEPT. The color change was independent of MHQ up to a certain concentration; above this the color change increased with the amount of MHQ. In general, the light activated materials were more color stable than the chemically activated materials. The color change was not affected by pH, but decreased when the oxygen had been removed from the storage water. The test for color stability using UV-light irradiation gave results for proprietary resins that were not correlated with the results obtained by water storage.