It is now known that nanoparticles, when exposed to biological fluid, become coated with proteins and other biomolecules to form a 'protein corona'(1). Recent systematic studies have identified various proteins that can make up this corona, but these nanoparticle-protein interactions are still poorly understood, and quantitative studies to characterize them are few in number. Here, we have quantitatively analysed the adsorption of human serum albumin onto small (10-20 nm in diameter) polymer-coated FePt and CdSe/ZnS nanoparticles by using fluorescence correlation spectroscopy. The protein corona forms a monolayer with a thickness of 3.3 nm. Proteins bind to the negatively charged nanoparticles with micromolar affinity, and time-resolved fluorescence quenching experiments show that they reside on the particle for similar to 100 s. These new findings deepen our quantitative understanding of the protein corona, which is of utmost importance in the safe application of nanoscale objects in living organisms.