Azapropazone (1) is a non-steroidal anti-inflammatory drug (NSAID) whose chemical structure is markedly different From that of other agents in this class and challenges our understanding of structure-activity and structure-permeation relationships, Using a variety of experimental and computational techniques, we studied 1 for its molecular structure in the gas phase and non-protic polar solvents, protonation/deprotonation equilibria, tautomerism, and pH-lipophilicity profiles (octan-1-ol/H2O and dodecane/H2O). Other NSAIDs and model compounds were also examined For comparison. Due to its very low acidic pK(al), 1 exists in the physiological pH range as a zwitterion and as an anion. Some pharmacological implications of these findings are discussed