An experimental apparatus and test procedure to measure the sorption isotherm of moisture in porous building materials was developed and tested for its reliability and accuracy. The apparatus shows excellent potential for determining reliable moisture storage property data. The data are used in heat and moisture transport models. The apparatus consists of a small test chamber, with an external forced-air temperature and humidity conditioning system. The conditioning system supplies moist air to the test chamber at 20°C dry-bulb temperature over a range of 0.4 to 85 per cent relative humidity. The new system circumvents problems associated with the use of saturated salt-in-water solutions. Moisture content is determined gravimetrically without removing specimens from their conditioned environment. The mass is monitored in situ such that external ambient conditions have no effect on the data. The control of test chamber relative humidity and dry-bulb temperature is automated. Test results consisting of sorption isotherms and scanning curves for sugar pine, southern pine, and an exterior grade plywood are presented. The maximum uncertainty in moisture content measurement is 0.023 kg/kg-dry mass at 85 per cent RH. Results are compared to data from the literature. The study shows a need for a standard test method for measuring sorption isotherms. Minimum recommendations for such a standard test method are included. A comparison of scanning data to an analytical model from the literature show that further investigation of scanning is needed. The present method is well suited for such a study. / Master of Science