Stability of the color images produced by full-color Cycolor copiers was studied. Test pieces were prepared by copying the Macbeth Color Checker and Test Chart No.5-1 SEPJ in full size with Cycolor machines. An accelerated dark stability test was carried out in an incubator kept at 60-80°C and 60% RH, while light stability tests consisted of irradiation with a Xe light Fade Meter, fluorescentlamps or a north skylight. The degrees of image deterioration were evaluated by the change in color expressed in terms of color difference in CIELAB. Whether the deterioration is caused by discoloration or fading is easily judged by inspecting the ratio of the components in color difference. According to our tests, Cycolor images exhibited an obvious instability for dark and light storages. The degree and the behavior of deterioration varied for each patch and depended on test conditions. Light irradiation resulted in a remarkable deterioration consisting of discoloration and fading accompanied with a yellowing of the paper base. In contrast, the accelerated dark stability test, which caused a less degree of deterioration than the light stability test, brought about only discoloration.