Surface pressure isotherms of poly(lauryl methacrylate) (PLMA) spread monolayers at the air-water interface show that the thermodynamic state of the polymer is poorer than Theta conditions. Neutron reflectivity has been used to determine the organization of the polymer at the interface. To obtain maximum information, four variations of PLMA were synthesized, each with different isotopic labeling, and these were spread on two different aqueous subphases, D2O and air contrast matched water. Reflectivities were recorded at two different surface pressures of 5 and 9 mN m(-1). Interpretation of these reflectivity profiles using the optical matrix calculation methods suggested a two-layer structure with the methacrylate backbone immersed in the subphase and the lauryl substituent protruding into the air phase but being penetrated to some extent by water. Direct analysis of the reflectivity data using partial structure factors allowed the dimensions, separations, and compositions of each of the layers to be determined. The backbone and substituent layers were describable by Gaussian distributions with standard deviations of 6 and 16 Angstrom, respectively, at the highest surface pressure investigated. Over the same surface pressure range, the near surface water layer had a hyperbolic tangential organization. Separations between layers obtained from cross partial structure factors remained relatively unchanged as the surface pressure increased. The methacrylate backbone-rich and near surface water layers were separated by 2 Angstrom, and the lauryl substituent-rich and water layer separation was ca. 6 Angstrom. Methacrylate backbone and side chain distributions were separated by ca. 3 Angstrom at 9 mN m(-1). These values were interpreted as indicative of complete immersion of the backbone and partial immersion of the lauryl substituents.