Analysis of water in vacuum systems presents large difficulties. Generation of known partial pressures is necessary to calibrate mass spectrometer response for quantitative analysis. For this purpose, two designs of leaks and their calibrations will be described. Two problems of offgas analyses dictated the development of special techniques. One study, examining comparative surface desorption, required rapid sample insertion and data acquisition. The other study, the monitoring of slowly changing offgas rates, required high precision over extended periods. Very constant, elevated temperatures of the entire vacuum system are required to keep the water partial pressure a function of current sample offgassing. Exposure of mass spectrometers to water can cause instabilities to their sensitivities to all gases. Methods of decreasing this effect through ionizer modification and of acquiring good data through special calibration procedures will be described.