Interfacial activity of decyl pyridinemonocarboxylates with or without the presence of decanol was studied in systems containing an excess of modifier and a constant ratio of extractant to modifier used in comparable amounts. The importance of appropriate procedures for reprocessing the interfacial tension data has been demonstrated for the estimation of the absolute and relative interfacial concentrations of extractant molecules at the interface. Decyl isonicotiniate has the highest interfacial activity when compared to that of decyl nicotiniate and decyl picoliniate. When alcohol is used in a large excess then the interface is mainly occupied by its molecules. In systems containing comparable amounts of decyl picoliniate and decanol, the co-adsorption of these two compounds occurs with mole fractions at the interface close to 0.4 and 0.6 for decyl picoliniate and decanol, respectively. It should, however, be pointed out that the determination of the mole fractions of these two compounds at the interface was possible only in a very limited region of concentrations corresponding to a domain of interfaci al tension ranging from 26 to 29 mN m-1a to whom the correspondence should be sent