We report the observation of a paramagnetic resonance in p(+)-type mesoporous silicon contacted with several molecules with Lewis base character. Adsorption of NH3, pyridine, acetone, ethanol, and water generates an isotropic electron spin resonance (ESR) signal at g = 1.9984 independent of temperature. By comparing this value with those concerning shallow donors in bulk silicon, the paramagnetic center is identified with electrons in the conduction band. In the case of ammonia, studied in detail, the signal grows with the amount of adsorbed molecules until an ESR line-shape transition from Lorentzian to Dysonian occurs. This change in line shape parallels the transition from a Langevin to Pauli paramagnetism due to the onset of electron gas degeneracy. The phenomenon is pressure dependent and completely reversible in a range of coverages of the order of the monolayer. (c) 2005 The Electrochemical Society.