The spatial distribution of protein molecules interacting with a planar polyelectrolyte multilayer was determined using neutron reflectometry. Staphylococcal nuclease sSNased was used as model protein that was adsorbed to the multilayer at 22 C and 42 C. At each temperature, the protein solution was adjusted to pD values of 4.9 and 7.5 to vary the net charge of the protein molecules. The multilayer was built up on a silicon wafer by the deposition of poly ethylene imine PEI , poly styrene sulfonate PSS , and poly allylamine hydrochloride PAH in the order Si PEI PSS PAH PSS 5. Applying the contrast variation technique, two different neutron reflectivity curves were measured at each condition of temperature and pD value. From the analysis of the curves, protein density profiles normal to the interface were recovered. Remarkably, it has been found that SNase is partially penetrating into the polyelectrolyte multilayer after adsorption at all conditions studied. The measured neutron reflectivities are consistent with a penetration depth of 50 at pD 4.9 and 25 at pD 7.5. Since SNase has an isoelectric point of pH 9.5, it carries a net positive charge at both pD values and interacts with the PSS final layer under electrostatic attraction conditions. However, when increasing the temperature, the amount of adsorbed protein is increasing at both pD values indicating the dominance of entropic driving forces for the protein adsorption. Interestingly, at pD 4.9 where the protein charge is relatively high, this temperature induced mass increase of immobilized protein is more pronounced within the polyelectrolyte multilayer, whereas at pD 7.5, closer to the isoelectric point of SNase, raising the temperature has mainly the effect to accumulate protein molecules outside the polyelectrolyte multilayer at the water interface. It is suggested that the penetration of SNase into the polyelectrolyte multilayer is related to a complexation mechanism. The complexation is essentially entropic in nature due to the release of counterions