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Extensive results are given on the effects of HCI and picric acid on the capacity of wool to absorb water. Both the effect of changing the relative humidity and the effect of changing the acid content of the fibers were studied. Less extensive results were obtained using H2SO4, oxalic acid, and Orange II acid and salt on wool, and HC1 on silk. Collateral measurements were also made on the swelling of human hair in picric acid solution and on the electrical resistance of wool fibers containing acid at various relative humidities.The absorption of acid decreases the capacity of wool to absorb water at low and moderate relative humidities. The efficiency with which the absorption of water is reduced depends on the identity of the acid.At the highest relative humidities the acids divide into two groups. In the case of the first group, of which picric acid is an example, the effect of acid on the absorption of water is the same as at lower humidities. In the case of the second group, of which HCI is an example, there is a second mechanism of interaction, which tends to increase the absorption of water and competes with the mechanism causing the decrease at lower relative humidities. The second mechanism may become dominant, causing a net in crease. This has been shown by direct measurement and by analysis of swelling measurement.The mechanism of interaction decreasing the absorption is identified as competitive absorption of acid on primary bonding sites for water. The mechanism of the interaction increasing absorption has been tentatively identified as osmotic absorption of water resulting from mobility of absorbed anions at the highest relative humidities. It is also possible that anion hydration is important in some cases.