Developing new functional biomaterials requires the ability to simultaneously repel unwanted and guide wanted protein adsorption. Here, we systematically interrogate the factors determining the protein adsorption by comparing the behaviors of different polymeric surfaces, poly(ethylene glycol) and a poly(phosphoester), and five different natural proteins. Interestingly we observe that, at densities comparable to those used in nanocarrier functionalization, the same proteins are either adsorbed (fibrinogen, human serum albumin, and transferrin) or repelled (immunoglobulin G and lysozyme) by both polymers. However, when adsorption takes place, the specific surface dictates the amount and orientation of each protein.