The temperature dependence of the matric potential of soil water was investigated as part of a study of the effects of climate change on soil physical properties. Undisturbed samples of topsoils were collected in cylinders. The pore water suction near the center of each cylinder was measured using two miniature tensiometers. The temperature of the samples was varied in the range of 10°C to 40°C. Three different phenomena were observed. The first was a slow drift to increased pore water suctions with time. This was attributed to the rearrangement of soil particles and has been described as the age-hardening or thixotropic effect. The second was the appearance of pressure transients when the temperature was changed stepwise. These were such that a step increase in temperature produced a rapid reduction of pore water suction that decayed during a period of hours. This was attributed to pressure changes of gas bubbles entrapped within water-filled pores-these pressure changes being transmitted to the pore water. The third (or TISSI) effect was a linear increase in pore water suction with time that started at temperatures around 40°C. This increased rate of change persisted for at least three further days after the temperature was again reduced to 20°C. Tests with sand showed none of the above effects. Several hypotheses were tested in attempts to explain the third phenomenon. These included tests to determine if some soil components were dissolving at higher temperatures. However, neither electrical conductivity nor optical absorbance showed any effects at temperatures up to 45°C and times of up to 7 days. It is conjectured that the TISSI effect is associated with the formation of organic micelles, although this needs further research to confirm it.