X-ray surface scattering and interfacial tension measurements are used to study the solid-to-gas phase transition in soluble monolayers of F(CF2)(8)(CH2)(2)OH and F(CF2)(10)(CH2)(2)OH adsorbed at the water-hexane interface. X-ray reflectivity and diffuse scattering measurements determine the molecular ordering, the presence of domains, and the interfacial coverage of solid domains as a function of temperature. The temperature-dependent coverage can be analyzed by a functional form consistent with a critical transition proposed by theory.