The conclusions drawn from the results obtained are summarized as follows:(1) The sorption capacity of water vapour by primary cellulose is extremely smaller than that of secondary. It probably depends upon the differences in number hydroxyl group, the rate of degradation and the physical contruction between the both.The sorption velocity of water vapour by acetyl-cellulose is generally larger in secondary acetylcellulose than in primary. There appears to be two stages. One seems to follow to the equation dS/dt=K(Sm-St), i.e.K=1/t ln(Sm/Sm-St), where k is the sorption velosity constant, t is a time during sorption, Sm is the maximum sorption capacity and St is the sorption capacity at a given time, the other being given by the equation S-a log t=K, where K is sorptions velocity constant, S is the sorption capacity, t is a time during sorption and a is a costant depending on the kind of sample.(2) The relation between the sorption capacity of water vapour and the number of acetyl group of acetyl-cellulose seems to be expressed by the following equationx=my+Kwhere x is the sorption capacity, y is the difference between the theoretical acetyl group of tri-acetate and the measured acetyl group, K and m are constants depending upon temperature and humidity. This equation appears to be aplicable to the case of the relation between sorption capacity of water vapour and nitrogen content of nitro-cellulose.(3) The sorption capacity of water vapour decreases with rise of temperature. The effect of temperature on the sorption capacity is larger than that of water vapour pressure. The sorption isotherm is subjected by the equation x/a+bx-cx2=y, developed by S. Oguri from the Langmuirer's equation where x is relative humidity, a, b, c, are constants and y is the sorption capacity.(4) The sorption capacity of water vapour by cellulose derivatives is the largest in acetyl-cellulose: Acetyl-mannan, ethyl-cellulose and benzyl-cellulose diminish their capacity in order described. Benzyl-cellulose has the smallest capacity. For the sorption velocity and the sorption isotherm of these samples are also applicable the same equation as that preciously obtained for acetyl-cellulose. But benzyl-cellulose alone does not follow to the equation of sorption isotherm.