Transformations of natural images in the perceptually uniform CIELUV color space have been investigated with respect to perceptual image quality. To this end, digitized color images of four natural scenes were described on the basis of their color point distributions in the CIELUV color space. A new set of images was created by varying the chroma value of each pixel while the lightness and hue angle were kept constant. The chroma was changed in two different ways. (1) through the addition or subtraction of the same amount of chroma to or from the chroma value of each pixel; (2) through multiplication of the chroma value of each pixel by a constant. In thr-ee experiments, subjects judged the perceptual quality, colorfulness, and naturalness of the images on a ten-point numerical category scale. The results indicate that colorfulness is the main perceptual attribute underlying image quality when chroma varies. Colorfulness itself was Sound to depend on both the average chroma and its variability. In general, the subjects preferred slightly more colorful images to the original ones. The perceptual quality of the images was found to be closely related to the naturalness of the images.