The trivalent Erbium transition,4T11/24[/supI13/2 has an emission wavelength close to , 3 μm in a variety of host laser crystals. The fundamental 0-H vibrational stretch in water occurs at approximately the same wavelength. It is therefore expected that the absorption coefficient of water near 3 μm would be of the order of 104cm-1. In order to measure this value precisely with a transmission spectrometer, it is necessary to have a film with a thickness of the order of the reciprocal of the absorption coefficient of water. This implies a thin film of water of about one micron. It is also desirable to measure the water absorption coefficient in the infrared out to wavelengths as long as 12 μm. This would require cell thicknesses up to 20 μm. A special cell has been designed and constructed to meet these criteria of variable thickness, IR transmission and compatibility with high purity water samples. The cell is shown in cross section in Figure 1. It is designed to provide a variable thickness film of water and a liquid filled space adjacent to the film that acts as a reservoir. The IR transparent ZnSe windows are attached to the Invar holders which are separated by two Teflon gaskets and a Invar spacer with inflow and outflow ports. The two Invar holders are squeezed together, compressing the Teflon gaskets. This not only changes the film thickness, but provides a positive seal with the gaskets. The film thickness is adjusted to the desired value, the transmission of the water film is measured and the film thickness is verified after the transmission measurement. The sequence can then be repeated for different thicknesses of the cell.