Rate constants are reported for the buffer-independent decay of ten (E)-arylmethanediazontes in aqueous media at 25 degrees C, ionic strength 1 M (NaClO4), 4% 2-propanol, in the region of pH 4-12. The rate constants are proportional to hydrogen ion concentration at high pH and become pH independent in the low-pH region. Varying concentrations of oxyanion, amine, and hydrazine buffers over the range 0.05-0.2 M increased the pseudo-first-order rate constant for decay of the diazoates by less than 10%. The azide-water selectivities, k(2)/k(5), for partitioning of the benzyl groups in the decay of (E)-(3,5-bis(trifluoromethyl)phenyl)methanediazonate and the (3,5-bis(trifluoromethyl)phenyl)diazomethane are determined to be 0.20 and 0.21 M(-1), respectively, in phosphate buffered water and 0.27 and 0.26 M(-1), respectively, in 20/80 DMSO-water. It is concluded that these two reactants decompose, in these media, via a common free diazonium ion intermediate that is formed in the case of the diazoate upon unassisted N-O bond cleavage of the diazoic acid.