Poly(ethylene glycol) (PEG) surface grafts are used as surface coatings that impart resistance to nonspecific protein adsorption. In this work, brushes of poly(ethylene glycol), grafted to a cationic polylysine backbone and adsorbed onto mica, were investigated with the surface forces apparatus, providing direct measurements of forces and refractive index via thin-film interferometry, Surface force isotherms were measured for different molecular architectures. The temperature dependence of the film thickness was also studied. The observed interaction force is predominantly repulsive and nearly elastic and can be described by polymer-brush scaling theory. Comparisons between polymer-polymer and polymer-hard-wall compression isotherms show a significant interpenetration of the two polymer films.