The denaturation of human serum albumin (HSA) upon interaction with the surfactant sodium dodecyl sulphate (SDS) was examined by measuring the diffusion time of fluorophore (RITC) tagged HSA under near single-molecule conditions using fluorescence correlation spectroscopy. The diffusion time showsfour distinct regions as a function of SDS concentration, which corresponds to (I) opening of the tertiary structure, (II) non-specific SDS aggregation, (III) opening of the secondary structure, and (IV) aggregation of SDS around the secondary structure. Diffusion time increases from 383 ls for the free protein to 1002 ls forthe SDS bound protein, which leads to an effective increase in the hydrodynamic radius by a factor of about 2.6