The relative effectiveness of deoxyribonucleoside diphosphates, ribonucleoside diphosphates and phosphorylated B vitamins as substrates of the nucleoside diphosphatase of Golgi membranes have been determined. The substrates have been classified by cluster analysis. These findings suggest that a base ketone group and the 3'-OH of the substrates are important in promoting catalytic effectiveness, possibly by promoting hydrogen bonding during enzyme-substrate complex formation.