The acetylation of commercial vinylon fiber was studied by using the three kinds of acetylation methods for poly (vinyl alcohol). The degree of formalization of the vinylon fiber used was 27.0 to 29.0mol%. 1) The vinylon fiber was acetylated with a mixture of acetic anhydride and pyridine at 105 to 110°C. About 90% of the residual hydroxyl groups of vinylon was acetylated both in pretreated and untreated vinylons. The degree of acetylation of the pretreated vinylon was slightly lower than that of the untreated one. It is supposed that the lower degree of acetylation of the pretreated one is due to the partial recrystallization. The acetylated product is insoluble in the acetylating mixture. 2) The untreated vinylon was acetylated with an acetic anhydride containing sodium acetate under refluxing. The similar results were obtained as those in the case of the mixture of acetic anhydride and pyridine. 3) The vinylon treated at 110°C in water was acetylated with a mixture of acetic anhydride, acetic acid and sulfuric acid. No residual hydroxyl group was observed in the acetylated product, while the degree of formal linkage decreased. The acetylated products obtained in a certain concentration of sulfuric acid are soluble in the acetylating mixture which is stored for a certain period of time.<br>The commercial vinylon fiber was treated in water containing sulfuric acid (20g/100cc) at 70°C for 40min., in order to remove the intermolecular formal linkages (Kawase's method), resulting in a decrease of the degree of formalization by 1.4mol% (specimen 1). The poly (vinyl alcohol) fiber was formalized in the lower concentration of formaldehyde (0.19%) than in the usual condition to avoid the occurrance of the intermolecular formal linkage. The degree of formalization of this fiber was 26.6mol% (specimen 2). The residual hydroxyl groups of specimens 1 and 2 were completely acetylated with the mixture of acetic anhydride and pyridine, and the acetylated products were soluble in the acetylating mixture. The authors suggest that the results of the acetylation with the mixture of acetic anhydride and pyridine and with the acetic anhydride containing sodium acetate are explained by the high orientation of poly (vinyl alcohol) molecule and the crosslinkage formed by intermolecular formalization in the commercial vinylon, and is supposed that the disappearance of crosslinkage in the specimens deformalized with sulfuric acid is responsible for the results of the acetylation with the mixture of acetic anhydride, acetic acid and sulfuric acid.