Evaluation methods based on dispersive kinetics were applied to derive kinetic parameters for the rearrangement of differently a-substituted nitronic acids generated by flash photolysis (lambda(inc) = 347 nm) of o-nitrobenzyl esters. The rearrangements were performed in polymer matrices of different chemical nature : poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), poly(styrene) (PSt), and poly(dimethylsiloxane) (PDMS). A nonexponential decay behavior was observed in rigid and highly viscous matrices which can be characterized by the average rate constant nu and the dispersion factor sigma. It was found that the apparent activation energy of the average rate constant of the unimolecular reaction is inherent to the nitronic acid and depends, if at all, only slightly on the chemical nature of the matrix and on the mobility of the matrix. However, the mobility of the matrix exerts a strong influence on the dispersion factor a which is a measure of the width of the Gaussian distribution of individual energy barrier heights. sigma decreases with increasing temperature but approaches zero (transit from dispersive to nondispersive, i.e., first-order kinetics) at temperatures definitely higher than T(g). The more pronounced this effect is, the bulkier is the substituent at the ct-position of the nitronic acid. The activation entropy DELTAS(double dagger) was found to be negative in all cases, thus substantiating the existence of a formerly postulated cyclic intermediate. DELTAS(double dagger) depends only slightly on the chemical nature of the matrix but significantly depends on the chemical nature of the nitronic acid : in the case of the unsubstituted nitronic acid, DELTAS(double dagger) is much lower than in the cases of the substituted ones. This difference is thought to be due to hydrogen bonding which is only feasible in the case of the unsubstituted compound.