Osmolytes are substances that affect osmosis and are used by cells to adapt to environmental stress. Here, we report a neutron reflectivity study on the influence of some osmolytes on protein adsorption at solid-liquid interfaces. Bovine ribonuclease A (RNase) and bovine insulin were used as model proteins adsorbing at a hydrophilic silica and at a hydrophobic polystyrene surface. From the neutron reflectivity data, the adsorbed protein layers were characterized in terms of layer thickness, protein packing density, and adsorbed protein mass in the absence and presence of urea, trehalose, sucrose, and glycerol. All data point to the clear effect of these nonionic cosolvents on the degree of protein adsorption. For example, 1 M sucrose leads to a reduction of the adsorbed amount of RNase by 39% on a silica surface and by 71% on a polystyrene surface. Trehalose was found to exhibit activity similar to that of sucrose. The changes in adsorbed protein mass can be attributed to a decreased packing density of the proteins in the adsorbed layers. Moreover, we investigated insulin adsorption at a hydrophobic surface in the absence and presence of glycerol. The degree of insulin adsorption is decreased by even 80% in the presence of 4 M of glycerol. The results of this study demonstrate that nonionic cosolvents can be used to tune and control nonspecific protein adsorption at aqueous-solid interfaces, which might be relevant for biomedical applications.