The pK of an acid-base indicator is dependent upon the polarity of the environment in which it is measured. This will be true for anisotropic systems, such as spread monolayers, as well as for isotropic media. It is useful, therefore, to have some reference for medium polarity when comparing acid-base behavior in monolayers and other environments. The fluorescence intensity of the coumarin probe, 3-hexadecanoyl-7-oxy-coumarin (HOC), has been deterMined in monolayers of L-alpha-dioleoyl phosphatidylcholine and 1,2-dioleoyl-sn-glycerol as a function of subphase pH. For reference, parallel measurements were carried out with a short-chain coumarin analog in ethanol water mixtures. As the corresponding monitor of local polarity, the time-resolved fluorescence of nitrobenz-2-oxa-1,3-diazol has been determined in both the monolayers and the solvent mixtures involved in acid-base indicator studies. The pK of HOC in the lipid matrix at the gas-water interface was compared to that in ethanol-water mixtures; it is found that pK of this indicator is shifted to higher values in both types of monolayer relative to the pK in isotropic media of comparable polarity. The dependence of both polarity and indicator pK on molecular organization in these lipid systems is discussed. The variations in pK provide a method for estimating the difference between pH in the interface and that of the subphase.