A nonreactive spreading protocol for measuring contact angles of buffered water droplets on low-energy surfaces is described. The protocol consists of immersing the sample (prior to contact-angle measurement) in a buffer solution of the same pH as that of the buffered water droplet that will be used to measure the contact angle. Contact-angle titration data acquired using this protocol for acidic self-assembled monolayers of alkanethiolates on gold exhibit a smooth transition in contact angle between plateau regions at low and high pH. This is in contrast with data acquired for identical monolayers using the more common reactive spreading protocol, for which a plateau region could not be obtained at high pH. It is postulated that the buffer pretreatment leaves the surface id a partially deprotonated state even though the surface is not macroscopically wet; this postulate is supported by infrared reflection-absorption spectroscopy data which show a conversion from protonated to deprotonated carboxylate in a dry acid-containing monolayer after exposure to a basic buffer solution. The nonreactive spreading protocol has proved particularly useful for deriving equilibrium parameters, e.g. pK(A) values for surface acid groups, from contact-angle titration data. Mixed monolayers of 11-mercaptoundecanoic acid with the alkanethiols nonanethiol, decanethiol, undecanethiol, and dodecanethiol exhibited well-defined contact-angle titration curves from which pK(A) values for acid dissociation in the monolayers could be obtained. A strong dependence of pK(A) on alkanethiol chain length was observed, with pK(A) values ranging from approximately 6.5 for a mixed monolayer with nonanethiol to approximately 11.5 for a mixed monolayer with dodecanethiol. The latter value reflects a shift of 6.7 pK units relative to the pK(A) for butyric acid (a representative alkanoic acid) in bulk, water. The pK(A) shifts are interpreted in terms of two physical processes, one involving solvation of carboxylate anions in the monolayer microenvironment and another involving interfacial potentials at the monolayer-solution interface.