We established a highly sensitive quantitative analytical method for chondroitin/dermatan sulfates by LC/MS method. By this method, the unsaturated disaccharides produced after the enzymatic digestion of chondroitin/dermatan sulfates can be determined in the amounts as low as 0.5 pmol levels. The use of tetrabutylammonium hydroxide as an ion-pair reagent for LC/MS allowed us to separate unsaturated 4-sulfated disaccharide and unsaturated 6-sulfated disaccharide. Furthermore, the peak areas of unsaturated disaccharides were increased almost 10 times by the postcolumn addition of acetonitrile. We applied this LC/MS method to the analyses of unsaturated disaccharides from chondroitin/dermatan sulfates in the tissues sections on glass slides, which were prepared from MethA tumor-bearing mice. This method brought about considerable reduction in the time distance from sample collection to preparation of analytical results.