We describe a non-destructive, versatile, and accurate method for measuring the thickness of silicon membranes, as used for example in micromachined pressure sensors. The method makes use of interference fringes observed in the transmission of infrared light through a membrane. Thicknesses up to about 50 mu m for 1 X I mm membranes and 150 mu m for 10 X 10 mm membranes can be measured with an accuracy of about 2%.