Chemical recognition by base complementarity in DNA and RNA is strictly related to their stereochemical order. The way in which this high stereoregular order has been achieved in a prebiotic world is not fully understood yet. More primitive systems that display complementary base recognition as a prerequisite to information and, eventually, self-replication might represent a possible route. This study investigates phosphatidylnucleosides bearing complementary bases, adenine and uridine, that can mutually recognize each other, giving mixed structures with features characteristic of complementary base pairing. Dioleoylphosphatidyl derivatives of adenosine (DOP-adenosine), uridine (DOP-uridine), and cytidine (DOP-Cytidine) have been studied at the water-air interface as a function of pH and subphase composition. When monovalent cations (Li+, Na+, and K+) are dissolved in the subphase, the phosphatidyl derivative monolayers show expansion or compression depending on the cation nature. In particular DOP-adenosine shows a preferential interaction with Li+. The properties of mixtures of the DOP-adenosine/DOP-uridine complementary bases mere investigated and compared to those of the non-complementary bases (DOP-adenosine/DOP-cytidine) The results indicate a preferential interaction in a hydrophilic environment only for complementary nucleophospholipids at physiological pH, suggesting that the specific interfacial orientation of the phospholiponucleoside imposed by the interface promotes the molecular recognition between the two complementary bases in a way that resembles the Watson-Crick pairing In natural nucleic acids. Moreover, mixed monolayers of adenosine-uridine derivatives show a minimum of the free energy of mixing for DOP-uridine rich mixtures (around the DOP-adenosine/DOP-uridine = 0.2-0.3 mole fraction) close to the stoichiometry of the trimeric adduct (uridine)(2) adenosine that forms in highly concentrated solutions of uridine and adenosine, where adenosine displays simultaneously the Watson-Crick and the Hoogsten hydrogen bond patterns.