Monolayers of Stober-silica nanoparticles(ca. 40 nm diameters)at the water-air interface have been studied in a Wilhelmy film balance. Scanning angle reflectometry and molecular dynamics computer simulation have been used to assess the contact angles of the particles. Our results indicate that the traditional film balance method of contact angle determination overestimates the real contact angles even in the lower range of particle hydrophobicities.