The theoretical formalism for the treatment of magnetic resonance in the presence of nonequilibrium chemical reactions is presented. It is applied to the study of fast transient chemical reactions by means of NMR Fourier experiments. The main emphasis is put on the analysis of the lineshape. It is found that for transient reactions characteristic lineshapes occur which are qualitatively different from those well known for dynamic chemical equilibria. Experimental results using fast stopped-flow mixing techniques are described. The kinetic deuterium isotope effect in the hydrolysis of methyl formate is studied. © 1979.