The humidity sensing mechanism of the composite material of nanometer barium titanate and polymer quaternary acrylic resin (QAR) is discussed in terms of d.c. property, dielectric loss and thermally stimulated depolarization current (TSDC). Various conducting carriers in different humidity ranges are identified by the direct current curves. The dielectric loss may be caused by the polarizations of dipole orientation and interface charge. The TSDC patterns show that the active energy and relaxation time of the dipoles in the composite materials take continuous, rather than discrete, values. (c) 2007 Elsevier B.V All rights reserved.