[en] Modern analytical techniques now make it convenient to measure more than one dependent variable at any point of a kinetic run. Modern data analysis makes it equally convenient to treat integrated rate equations of almost any mathematical complexity. These advantages have been exploited to formulate a general procedure for the kinetic analysis of isotopic and/ or optical labelling experiments. Its principal goal is the rigorous and quantitative analysis of alternative reaction mechanisms. Its use is illustrated in outline form, by application to the two unimolecular reactions which originally stimulated its development: the oxygen scrambling of acetyl peroxide and the degenerate rearrangements of hexa-1,5-diene. Extensions to bimolecular reactions and to reactions that exhibit appreciable isotope effects will be described elsewhere. (K.A.)