We have examined the adsorption of a nonionic surfactant, penta(ethylene glycol) monododecyl ether (C12E5), at the poly(methyl methacrylate) (PMMA)-water interface using spectroscopic ellipsometry. The solid PMMA surface was deposited by spin casting an ultrathin film onto a freshly cleaned silicon wafer. Measurements by both spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) showed that the thin PMMA film was uniform with no prominent structural features on the surface. The adsorption of C12E5 at the solid PMMA-aqueous solution interface was studied using a specially designed cell with a fixed angle of incidence of 75 degrees, and the measurements were made over a wide concentration range around the critical micellar concentration (cmc). It was found that the adsorption is completely reversible and that there is no observable penetration of C12E5 into the PMMA. The adsorption was found to reach equilibrium well within seconds. Although spectroscopic ellipsometry cannot allow a reliable measurement of layer thickness as a result of coupling between refractive indices and layer thickness for ultrathin layers, the surface excess at a given concentration can be determined reliably. The limiting area per molecule at the cmc was calculated to be 50 +/- 3 Angstrom(2), in good agreement with the value obtained from a previous neutron reflection study.