Reports on temperature (T) effects on time domain reflectometry (TDR) measurements of soil water content (&thgr;) are contradictory and often exhibit conflicting trends. We imposed step T changes on sealed columns of four soils having variable &thgr;, while monitoring bulk apparent dielectric constant (or permittivity ϵb) and bulk electrical conductivity (&sgr;b) using TDR. Measured ϵb increased substantially with increasing temperature for one silt loam soil, for all &thgr;. For another silt loam soil and for an Oxisol, measured ϵb increased with increasing T at relatively low &thgr; but decreased with increasing T at higher &thgr;. For a sandy loam soil, measured ϵb decreased with increasing T for all &thgr;. The experimental results led to the hypothesis that TDR-measured ϵb is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric constant of bulk water with increased T; and (2) the increase in TDR-measured ϵb with increased T due to release of bound water. TDR-measured ϵb is thus dependent on solid surface area and wetness. Our results have implications for routine use of TDR in fine-textured and organic soils and potentially for microwave remote sensing of soil water status. ¿ 1999 American Geophysical Union