Ellipses representing the discrimination of small color differences in the CIE chromaticity diagram have been determined recently by several investigators using surface colors (paints and textiles). When the classical MacAdam ellipses are interpolated to the same color centers they are oriented in significantly different directions. The orientation of the ellipses determined by Wyszecki and Fielder, who, like MacAdam, used colored lights, agrees better with the surface-color results in most cases. The ellipses from the various surface-color studies agree well with each other in shape and orientation, but the relative sizes for different color centers do not agree.