The moisture contents of ten kinds of textile fibres in the form of specially prepared yarns were determined when the fibres were in equilibrium with air for a series of relative humidities and tem peratures. Data were obtained for both "desorption." and "ad sorption," the yarns being brought to equilibrium from a wet condition and a dry condition, respectively. The fibres studied were raw cotton, "purified" cotton, mercerized cotton, clothing wool, carpet wool, viscose rayon, cuprammonium rayon, raw silk, degummed silk and cellulose acetate. The temperatures ranged from 96° F. to 302° F. and the relative humidities ranged from 5% to 90% for temperatures below 212° F. and up to the maximum obtainable at atmospheric pressure above 212° F.The results are given in the form of graphs showing moisture content against relative humidity and also in the form of graphs in which the logarithms of the moisture contents at given relative humidities are plotted against the reciprocals of the corresponding absolute temperatures. The last show straight line relationships with changes in the slopes of the lines between 200° F. and 220° F.