6-O-Stearoylascorbic acid (ASC) and vitamin K-1 (2-methyl-3-phytylnaphthoquinone) behave as surfactant molecules and form stable monomolecular spreading films at the gas/water interface. This paper reports an investigation on the miscibility of these two amphiphiles in monolayers as a function of temperature and composition of the mixture. A thermodynamic theory is applied to the experimental results in order to obtain information on the intermolecular interactions occurring at the gas/water interface between the two components. Experiments show that the two components are fully miscible in all ratios and at all temperatures, with repulsive interactions that are probably due to the presence of branched methyl groups in the phytyl chains of vitamin K-1 and to the presence of a negative charge on the hydrophilic head group of ASC.