Hypothesis: The net surface charge of AlGaN/GaN structures, where AlGaN is in contact with the solution, is controlled by the pH-dependent protonation and deprotonation of the surface hydroxyl groups and possibly the electron-deficient surface electronic states. We hypothesize that atomic force microscopy (AFM) force measurements of ionic surfactant adsorption can reveal how the AlGaN surface properties vary with pH. Experiments: AFM force curves and images were used to probe the AlGaN/solution interface in water as a function of pH, and with added cationic surfactant cetyltrimethylammonium bromide (CTAB) or anionic surfactant sodium dodecylsulfate (SDS). Findings: The AlGaN/solution interface is negatively charged at pH 12, has an isoelectric point near pH 5.5, and is positively charged at pH values less than 5.5. Surfactant adsorption data suggests AlGaN surface is somewhat hydrophobic at acidic pH. Compared to gallium nitride (GaN), at pH 2, AlGaN has a lower charge density and hydrophobicity, but at other values of pH, the surface properties of AlGaN and GaN are similar. Crown Copyright (C) 2020 Published by Elsevier Inc. All rights reserved.