Aqueous dispersions of partially deacetylated α-chitin (PDACh) were prepared using various organic and inorganic acids in water under various pH and ionic strength conditions. The degree of PDACh nanofibrillation in the dispersions was investigated by measuring their light transmittances, nanofibrillation yields, and zeta-potentials, and by atomic force microscopy observations. Highly transparent dispersions, consisting of mostly individualized PDACh nanofibrils (φ = 3.4 ± 1.5 nm), were obtained when ascorbic acid and certain monovalent acids were used in water at pH 3.5. However, unfibrillated PDACh bundles were present in dispersions prepared with divalent and trivalent acids, probably because of formation of ionic cross-linkages between PDACh fibrils. When the acid concentration in the dispersions was adjusted to 0.04 M, those with higher dispersion pH exhibited higher nanofibrillation efficiency. Addition of excess acid inhibited efficient nanofibrillation of PDACh, owing to the high ionic strength that concomitantly occurred.