We have employed Langmuir monolayers of polydimethylsiloxane-polystyrene diblock copolymers as a model system for examining layers of tethered chains under good solvent conditions. The range of surface density accessible with this system coincides with the ranges reported in the literature for chains tethered onto solid substrates from dilute solution in good solvents. We have varied both the surface density (sigma) and the molecular weight (M) of the submerged polystyrene block independently, covering over a decade in each variable. Both the form of the segmental concentration profile and the variation of the layer height with sigma and M are in good agreement with numerical self-consistent-held (SCF) calculations. On the other hand, we do not observe precise agreement with the scaling predictions for strongly stretched chains, in contrast to some previous reports. Through measurements of the surface pressure (II), we present the first direct comparison between anchoring energies and differential chain energies in tethered layers. We fmd these to be of equal magnitude at the desorption transition. However, the chain energies reach values roughly an order of magnitude larger than can be accounted for by the osmotic interaction of the polystyrene segments in the good solvent. In addition, the dependence of II on sigma is more consistent with a model of soft spheres with hard cores than that of a semidilute polymer mesh. The hard core areas seem to be loosely related to R(g). We attribute these observations to a steric effect which limits lateral interpenetration of the submerged blocks. The sharp rise in II with sigma is of great practical importance as it limits the maximum surface coverage in this system, and may also do so for other systems.