Measurement of water vapor concentration or humidity is important in many areas of atmospheric research. Most existing instrumentation does not have adequate sensitivity or time response for measurements at high altitudes or in cold regions, or for flux measurements in the surface layer. The purpose of this research is to study the development of a high resolution infrared absorption sensor for fast-response measurement of water vapor, utilizing a liquid nitrogen-cooled lead-salt diode laser. In Phase I of this SBIR program, a multiple pass absorption cell design which has excellent alignment stability was demonstrated for application to water vapor measurements. A set of absorption lines was identified which can be used to achieve a very wide dynamic range sufficient to accommodate measurements of humidity under all expected conditions in the troposphere or lower stratosphere. Accurate measurements of the integrated line strengths for these lines were made in Phase I. The major objectives of the Phase II SBIR research are to study methods for simplifying the instrument design using infrared fiber optics and to enhance measurement accuracy by using a dual-beam method. A prototype humidity sensor will be assembled for laboratory and local field tests. These tests will include detailed evaluations of sensitivity, accuracy, and time response, including intercomparison with other instrumentation used for water vapor measurements in atmospheric or meteorological research.