In a systematic optimization process Jive sets of recent color difference data have been analyzed for commonalities. Adjustment of the X tristimulus values and application of a systematic, surround dependent S-L function was found to be beneficial in all cases. Other modifications of the CIE94 color-difference formula were found to bring improvements only in some cases and may be spurious. Application of what seem to be nonsystematic scale factors in a range of 0.78-1.38 improve correlation between calculated and visual color differences in all cases. After optimization, calculated color difference values explain between 80-90% of the variation in visual color differences. Some of the datasets are shown not to be well suited for formula optimization. Optimization in all cases by set, for three sets of data by quadrant in the a*b* diagram, and for one set by subset did not reveal any additional systematic trends for improvement. It appears that the basic structure of CIE94, with the recommended modifications, is a good approximation as a model for color-difference evaluation in the range from 0.5-10 units of difference. The model is surround dependent. A number of issues remain to be resolved. (C) 2001 John Wiley & Sons Inc.