A new technique, hydrodynamic fingerprinting, is presented for the characterization and study of expandable latex dispersions on the basis of their hydrodynamic behavior. In the hydrodynamic fingerprint the hydrodynamic size as measured by photon correlation spectroscopy is presented as a function of two colloidal state variables simultaneously, pH and p lambda, where p lambda, is the logarithm of the conductivity. Hydrodynamic fingerprinting is applied to two methyl methacrylate-ethyl acrylate latexes, one of which contains 2% acrylic acid. It is shown that hydrodynamic fingerprinting can be used to measure expansion properties of colloids, time effects in colloidal solutions, colloidal stability, and the differences in chemistry of the colloid-solution interface.