Radicals generated by photolysis of bis(1-methylethyl) diazenes (azoisopropanes) Me₂C(X)-N=N-C(X)Me₂, where X = -CN, -SCN, -SPh and -OAc, have been studied by means of spin trapping and radical scavenger techniques. The primarily photoproduced radicals of the type C(X)Me₂ were trapped by N-t-butyl-a- phenylnitrone, except for X = -SCN. In this case, thiocyanatyl radicals were trapped by N-t-butyl-a- phenylnitrone . Radicals generated by thermoiysis were all of the type 'C(X)Me₂. The diffusive separation process of a pair of photoproduced geminate radicals was investigated spectrophotometrically by following the disappearance of 2,4,6-triphenyl-3,4-dihydro-1,2,4,5-tetrazin-12H)-yl ( verdazyl ), in binary solvent systems of different viscosities. The reciprocal of the apparent zero-order rate constant with respect to verdazyl was found to be linearly related to the viscosity. This first-power relationship is briefly considered.