Perfluorocarboxylic acids and their derivatives are utilized industrially in materials like Scotchgard? and aqueous firefighting foams because of their high hydrophobicity. While much research is being done to develop methods to remove these compounds from the environment, little research has been done to characterize their air-water interface activity and structure. In this work, seven perfluorocarboxylic acids were studied, ranging in chain length from 8 to 14 carbons long. Surface pressure isotherms were measured with Langmuir-Blodgettry for each acid in order to determine their molecular footprint as well as any phase transition points, specifically at molecular areas of 500 ?2 and larger. The primary results show a chain length dependent trend in the phase transition position. The phase transition shows a decrease in surface pressure with an increase in molecular area as the chain length of the compound increases.