Aluminium nitride substrates were immersed in acid, basic solutions and deionized water for 1-120 h at room temperature. The corrosion rates are higher in basic solutions (NaOH and KOH) than those in acid solutions (CH3COOH, HCOOH, HNO3, HCl and H2SO4) and deionized water. The weight loss of AlN corroded in alkali aqueous reaches 70% and results in an increase in surface rough ness ranging from 10 nm to 7 mu m after 3 days corrosion. However, the weight loss in acid solution is only 1/700 of the alkali case. Violent chemical reactions between AlN and basic solutions were observed. Na2O, or Na2Al2O4.6H(2)O, is the intermediate product, and NaOH is a catalytic agent of the reaction. The surface morphology of the AlN etc hed by alkali ne solutions is coral-like in microscopic view and appears like hills. In contrast, only several atomic layers of AlN surface are etched off in acid solutions and in deionized water. The lightly etched surface is mirror-like and flat, and the shapes of the grains are visible under the microscope, as the corrosion rate of each AlN grain varies with different crystal orientations. Consequently, after etching in acid solutions, the resulting microscopic surface morphology looks like a map of a jigsaw puzzle.