A spectroscopic system based on vapor-phase absorption of infrared radiation produced by a diode laser is shown to be useful for measuring water vapor diffusion rates through materials in sheet form. Measurements of water vapor diffusion through four polymer membranes and membrane laminates show an excellent correlation with an existing water vapor transport apparatus that uses hygroscopic polymer-film capacitance sensors. The diode laser spectroscope eliminates sorption hysteresis artifacts present in the capacitance sensors.