Disk drives experience a wide range of temperature and humidity during storage and operation. Humidity influences media tribology through the effects of adsorbed water on the interaction of Zdol lubricant hydroxyl end groups with the carbon overcoat, We measured the effects of Zdol 4000 thickness and chemisorption on the water adsorption isotherm and surface energy. The potential distortion model of Adamson was employed to derive the film thickness at saturation from the isotherms and contact angles. The isotherm was surprisingly independent of the Zdol chemisorption and thickness up to 1.8 mm, because, in this range of Zdol thickness, the adsorbed water resides in pores which exclude the larger Zdol end groups. The water adsorption isotherm provides a sensitive probe of overcoat structure due the ability of water to reach pores in the overcoat which are inaccessible to Zdol, Above a Zdol thickness of 1.8 nn there is a jump in the water adsorption as additional adsorption takes place on the "free" hydroxyl groups. The mechanism for the previously observed decrease in Zdol lubricated media durability at low humidity is derived from the effect of adsorbed water on Zdol surface diffusion and chemisorption.