The surface forces apparatus was used to study the variation with relative humidity of adhesion between mica and rough gold films coated with either a hydrophobic or hydrophilic self-assembled monolayer (SAM). Pull-off forces, as a function of relative humidity, were measured between mica and gold coated with either n-octadecanethiol or 1-hydroxylunicosanethiol. For the alcohol-tenninated SAM, pull-off forces increase from 370 mN/m (dry conditions) to 845 mN/m (saturated conditions). For the methyl-terminated SAM, pull-off forces increase from 160 mN/m (dry conditions) to 643 mN/m (saturated conditions). A quartz crystal microbalance was used to obtain adsorption isotherms on the two different SAMs. Angstrom-level changes in adsorbed layer thickness are associated with marked changes in measured adhesion forces.