Technical Abstract: Tension infiltrometers are useful for measuring in-situ near-saturated infiltration rates in soils and fractured rock. Fluctuating ambient temperatures can cause dramatic pressure variations due to the presence of confined air inside the infiltrometer, thus affecting the intended pressure head to be applied to the soil or rock surface, and hence also the measured flux rate. This is especially true for low infiltration rates and long equilibration times. 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 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 significant reduced the undesired effects of temperature on tension infiltrometers results.