We have investigated the effect of submonolayer molecular adsorptions on the surface-plasmon spectrum of silver and aluminum surfaces using low-energy electron-loss spectroscopy. Plasmon energy shifts of about 200 and 500 meV for one-monolayer hydrocarbon coverage were observed on silver and aluminum films, respectively. In addition, a new loss peak appears upon adsorption, which we associate with a molecular electronic resonance. Its energy shifts with increasing coverage in the opposite direction to that of the plasmon. A classical dielectric model is presented in which the losses are attributed to two branches of a collective excitation in a system consisting of (adsorbate) oscillators on top of a semi-infinite metal surface.