The thermal denaturation of the A and B variants of bovine β-lactoglobulin has been examined using optical rotatory dispersion and sedimentation velocity techniques. An equation is derived which describes the time dependence of optical rotation in terms of parameters relevant to a system involving two consecutive first order reactions. Analysis of simulated and experimental curves according to the procedure of Guggenheim (Phil. Mag., 2 (1926) 538) suggests that two consecutive first order unfolding reactions dominate the early stages of the denaturation. Optical rotatory dispersion and circular dichroism analysis in the far ultraviolet region of the spectrum shows that there is a significant increase in the proportion of β-structure in the molecule on thermodenaturation. In addition, thermodenaturation causes an aggregation of the protein, the extent of which depends on both the time and temperature of heat treatment.