Tension infiltrometers have become popular for in situ measurement of the near-saturated hydraulic conductivity as a function of the soil water pressure head. Unfortunately, fluctuating ambient temperatures can cause dramatic pressure variations due to the presence of confined air inside the infiltrometer, thereby affecting the intended pressure head to be applied to the soil surface and hence also the measured infiltration rate. This is especially true for low infiltration rates and long equilibration times typical of low permeability materials, including unsaturated fractured rock. We developed a model to analyze the effects of temperature changes on the static pressure of the confined air volume within tension infiltrometers. The model was tested using several prototype infiltrometer designs in a computer-controlled variable-temperature room. Experimental results confirmed the model simulations. For example, predicted and measured changes in the pressure head in one experiment were 11.2 and 12.5 cm following a change of 18.5°C in the temperature. Using the model, we were able to optimize several infiltrometer designs that significantly reduced the undesired effects of temperature on tension infiltration results.