Moisture sorption isotherms of acid collagen fibers, CSs (pH 2.3-2.9) and CWs (pH 3.7-4.2), prepared from alkali-processed collagen fibers, Cs (pH 5.4-6.2), were measured. The differences in the Cs, CSs, and CWs in moisture content, heat of moisture sorption, entropy change of moisture sorption, and parameter constant value of the Henderson and the Chung and Pfost equations were investigated. At a water activity above approximately 0.3, moisture content increased in the order CSs, CWs, and Cs. The lower order of Cs, CWs, and CSs in heat of sorption at a low moisture content indicated a decrease in linkage force strength between the water molecule and collagen because of a decrease in the change of the carboxyl group due to the preparation at lower pH. The structural stability of CSs toward moisture sorption was less than those of the Cs and CWs.