ABSTRACT BCS (Biopharmaceutics Classification System) and BDDCS (Biopharmaceutics Drug Disposition Classification System) were proposed as tools for classifying drugs into four categories. Both systems consider the solubility as an important characteristic for the classification of compounds in drug development and in vivo disposition prediction. Although some results of drug solubility can be found in the literature, the aforementioned characteristic is not entirely clear when considering didanosine (ddI). Based on that, the solubility of ddI was evaluated using equilibrium and intrinsic dissolution methods. For the equilibrium method, excess amount of ddI was added to each media until obtaining a supersaturated solution and the mixture was submitted to agitation at 37 °C. For the intrinsic dissolution method, the drug was compressed into the Wood's apparatus matrix and subjected to dissolution in each media with agitation at 37 °C. The results obtained from the equilibrium method indicated that it was necessary 139.37 mL of pH 1.2 media, 87.72 mL of pH 4.5 media, 12.54 mL of pH 6.8 media, 5.03 mL of pH 7.5 media and 7.65 mL of purified water for drug solubilization. Furthermore, a very fast intrinsic dissolution rate (IDR) was obtained for each media: 0.1 mg/min/cm² (pH 1.2), 0.2 mg/min/cm² (pH 4.5), 0.2 mg/min/cm² (pH 6.8), 0.1 mg/min/cm² (pH 7.5) and 0.1 mg/min/cm² (purified water). Based on these results, ddI can be considered as a highly soluble drug for both equilibrium and intrinsic dissolution methods.