A technique has been developed using an atmospheric pressure ionization mass spectrometer to measure the submonolayer equilibrium adsorption properties of moisture on tubing surfaces. This technique has been used to generate isobar data for moisture on 316 stainless steel surfaces with various commercially available surface treatments. Both a dissociated absorption model based on a Temkin energy distribution and a semiempirical adsorption model have been fit to the data. Though differences in the isobar data were clear, moisture absorption did not vary strongly with tube surface treatment. The implications of these results on tubing dry-down dynamics were confirmed by running controlled dry-down tests from an initial level of 200 ppb using an inlet gas at 0.2 ppb. The ranking of dry-down times (fastest to slowest) was the same as predicted by the isobar data. The difference between the dry-down time of the fastest and slowest responding tubes was approximately a factor of 4.