The photoreduction of 1.3-, 1.4-, 1,5-, 1,8-, and 2,3-dinitronaphthalene (DNN) as well as of 4,4'-dinitrobiphenyl and 2,7-dinitrofluorenone by 1,4-diazabicyclo [2.2.2] octane (DABCO), triethylamine (TEA), and diethylamine (DEA) in argon-saturated acetonitrile and mixtures with water was studied by spectroscopic and conductometric means after nanosecond 354 nm pulses. The rate constant for quenching of the triplet state of the dinitroarenes by TEA is k(q) approximate to 1.6 x 10(10) M-1 s(-1) in acetonitrile and markedly smaller on an addition of water. The secondary transients with maxima at 380-550 nm in the presence of TEA or DEA at concentrations of <1 mM and of DABCO are ascribed to radical anions of the dinitroarenes, formed via photoinduced electron transfer to the triplet state. With TEA at higher concentrations, in addition a delayed radical formation, involving the a-aminoethyl radical as precursor, was observed throughout. The radicals subsequently undergo a termination reaction yielding the corresponding nitroso compounds as major primary products; the process is substantially slowed in the presence of water (less than or equal to5 M). For 1,4-DNN a longer lived intermediate is formed. It is attributed to the dianion, which is also observable in alkaline 2-propanol. The two photoinduced reduction reactions of the dinitroarenes by TEA and the subsequent radical disproportionation are supported by time-resolved conductometric measurements. The conductivity signals have generally a much longer half-life than the absorbing intermediates. The mechanism and the effects of substitution and concentrations of TEA, oxygen, and water are discussed.