The adsorption of water on the surfaces of ultradispersed diamond UDD.powder obtained from explosive detonation and modified by treatment in different gases under high temperatures is investigated by FTIR method. It is found that UDD adsorbs atmospheric water quickly and in the IR region of OH stretching vibration of water, a broad asymmetric band envelope centered at 3412 cmy1 with shoulders at 3575 cmy1 and 3240 cmy1 appears soon after the sample pellets are exposed to air. The amounts and the initial rates of water of adsorption are different for UDD modified by different gases. Based on the experimental results three forms of adsorbed water are suggested. q1998 Elsevier Science B.V. All rights reserved.