The behavior of moisture gain and equilibrium moisture content (EMC) was determined for 30 drug substances at relative humidities (RH) ranging from 11% to 93%. Based on the results, the drugs were categorized into different classes, following the classification system proposed by Callahan et al. About 23 fell under Class-I (nonhygroscopic), four under Class-II (slightly hygroscopic), and three under Class-III (moderately hygroscopic). Most of the Class-III drugs converted from solid to liquid state at > 75% RH, and the moisture increase was more than 40% above 90% RH. However, the moisture increase was comparatively much smaller at humidities < 40%-50%. Hence, no drug could be categorized in Class-IV (very hygroscopic), where gain of moisture is generally higher even at lower humidities. The results were correlated to the statements given in the compendia on hygroscopicity, and also the values of loss on drying (LOD). The study suggests that there is a need to rationalize the pharmacopoeial information.