Molecular layers of oleate at a fluorite crystal surface were prepared by spontaneous adsorption from alkaline aqueous solutions of sodium oleate. The adsorption density of such oleate layers was determined by Fourier transform infrared internal reflection spectroscopy (FT-IR/IRS). Advancing and receding contact angles were measured for water drops on these carboxylate layers using the sessile-drop technique. The effect of surface heterogeneity (as established by the extent of oleate adsorption) on contact angle hysteresis for varying drop size is demonstrated. It was observed that both the contact angle hysteresis and contact angle/drop size relationship depend on the extent of deviation of the adsorbed layer from an ideal well-organized monolayer. Experimental results support the position that it is of particular importance to examine both the advancing and receding contact angles with varying drop size whenever the sessile-drop technique is used to describe the nature of heterogeneous surfaces, including those prepared by adsorption of organic molecules. It was also found that the advancing contact angles for water drops are essentially the same for an equivalent level of adsorption of oleate and stearate at a fluorite surface.