The purpose of this study was to investigate the interaction between phosphatidylcholine (PC) and ergosterol (ERG) in monolayers at the air-water interface. We measured the surface tension of pure and mixed lipid monolayers at 22 degrees C using the Langmuir method with a Teflon trough and Nima 9002 tensiometer. Surface tension values were used to calculate pi-A isotherms and determine molecular surface areas. Interactions between PC and ERG caused significant deviation from the additivity rule. The theory described in this paper was used to determine a stability constant reflecting the area occupied by one molecule of the PC-ERG complex and the complex formation energy (Gibbs energy). (C) 2019 Elsevier Ltd.