Steady-state flow of gas through a vacuum process after some initial transient time produces a stable composition indicated by gas analysis. But commonly during vacuum processing, rapid changes in gas composition regularly occur; sampling such processes and analyzing the gas with a residual gas analyzer (RGA) can show a long time constant for gas species that have a strong interaction with the walls. A well known case is sampling gases with water vapor as a component of the gas. When onset of the sampling flow occurs, water vapor is adsorbed by the sampling system walls from the flowing gas and the water vapor partial pressure arriving at the RGA is lower than the process value. As flow continues, the wall is saturated and in equilibrium with the flowing gas so that a more accurate value is measured for water vapor. When the process gas is pumped away or water vapor level decreases, an exponential tailing of the water vapor signal occurs as the walls degas. This paper addresses the details of this transient adsorption-desorption process in gas sampling tubes related to changing species concentrations by modeling the gas-surface interaction. Some insights from the model for species equilibration time include the role of species adsorption energy, Î"H, with the surface material, the effect of concentration of the absorbing species in the gas and total pressure of the flowing gas on equilibration time and finally the temperature dependence of the equilibration time. An additional surface effect in RGA usage is the change in surface potential with changing adsorbed or surface reacted gases on ion source and rod surfaces and the filament. Changes in sensitivity can occur when gas is changed from a reducing or neutral gas to an oxidizing gas. This sensitivity change is slowly reversible by returning gas flow to a reducing or neutral gas. Examples of this effect and methods to minimize the effect are presented.