In photochemically, electrochemically and thermally initiated decomposition, compounds containing the azo group (1RN=N2R) usually form reactive, short-lived radicals 1R• and 2R•, which corresponds to synchronous splitting. As examples, aliphatic azo compounds, 1-aryl-2-alkylazo compounds, azophosphonates, azosulfonates, azosulfones, azosulfides, triazenes, pentazadienes, hexazadienes, acyl- and diacylazo compounds, diazonium salts and cyclic azo compounds have been studied. 1R• and 2R• were characterized by means of the EPR spin trap technique. These reactive radicals may abstract hydrogen from the solvent or any neighborhood functions, or dimerize (1R—2R). Some azo compounds trap 1R• and 2R• and thus form the corresponding hydrazyl radicals. In cathodic reduction, prior to the initiation of decomposition, corresponding anion radicals centered on the azo group were observed which have considerable stability if 1R and 2R are conjugated structures. © 1998 John Wiley & Sons, Ltd.