There are theoretical difficulties in using surface-pressure-area data to calculate the extent to which a water-soluble surfactant penetrates into an insoluble monolayer. The various theoretical treatments that have been proposed require either simplifying assumptions or approximations or additional data. Of these treatments, that of Hall (Langmuir 1986, 2, 809-812) is the only one that is free of theoretical objections, but it does need values for the equilibrium spreading pressures of the monolayer on the various surfactant solutions. Such measurements are reported here for several systems, and in conjunction with new and previously-published surface-pressure-area data have been used to calculate the equilibrium penetration of the surfactant into the monolayer. Practical difficulties in applying these procedures are discussed. In particular, calculations that rely on data from the experimentally difficult region at low monolayer areas, as the Hall treatment generally does, often yield unsatisfactory values for the extent of penetration.