The adsorption of the flexible, linear, and nonionic homopolymer poly(vinyl pyrrolidone) from water and from 1,4-dioxane onto pyrogenic silica was studied. Results are reported for the adsorbed amount as a function of adsorption time, molecular mass, and molecular mass distribution (polydispersity). It is found that the adsorption of fractionated samples can be qualitatively explained by the recent theory of Scheutjens and Fleer. However, the influence of the solvent type is larger than predicted by this theory, and an extension of the model to account for this influence is suggested. The polydispersity effects encountered in adsorption isotherms are satisfactorily accounted for by a theory published by Cohen Stuart, Scheutjens, and Fleer.