ATR-IR spectroscopy is proposed as a versatile technique for the investigation of reaction kinetics in solution, particularly in aqueous solution. Subtraction of the solvent spectrum and thus, quantitative absorption measurements are possible. In contrast to thin film transmission spectroscopy, thermostatting is feasible. The high information content of IR spectra demands high resolution spectra measured at many wavenumbers, and for kinetic investigations, series of spectra are measured. A multivariate non-linear least-squares fit is proposed which effectively overcomes the difficulties related to the large number of data and parameters to be handled. As a test for the method, the base catalyzed hydrolysis of trimethyl phosphate in aqueous solution has been investigated.