A method is presented for estimating reaction rate constants very rapidly from on-line short-wavelength near-infrared measurements. It can deal with a strong spectral overlap between the individual spectra of the species in the reacting system. For this an iterative curve-resolution method is combined with a nonlinear fitting procedure. This procedure also makes it possible to assess the quality of the estimated reaction rate constants. Simulations show that the method can be applied in cases with extreme spectral overlap of the individual spectra of the species. The procedure is applied in practice to an example with a strong spectral overlap. Reaction rate constants of the two-step epoxidation of 2,5-di-tert-butyl-1,4-benzoquinone were estimated. This method can lead to the application to on-line monitoring reaction rate constants.