The molecular motions in acylated chitins, including acetyl, propionyl, butyryl, caproyl, capryl, lauroyl, and benzoylchitin, were investigated by ESR measurements with 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl (TEMPO) as a spin-probe. Introduction of a longer side chain into the chitin molecule resulted in a remarkable lowering of the temperature at which the separation between the outer two extrema of the ESR spectrum decreases adruptly (TN). Below and above TN, two kinds of motional process were observed, one at a lower temperature having a smaller activation energy and the other at a higher temperature with a larger activation energy. These processes are a small scale motion such as rotational oscillation and a large scale motion such as rotation, respectively. The activation energy of the latter process decreased with increasing chain length, except for acetylchitin. Fully acetylated chitin films were anomalous with respect to the value of the activation energy.