Rate constants, k(0), for the buffer-independent decomposition of four primary (E)-alkanediazoates and one primary (Z)-alkanediazoate in aqueous media at 25 degrees C, ionic strength 1 M (NaClO4) are reported. Between pH 4 and 12 the plot of log k(0) against pH is biphasic with a hydrogen ion-dependent reaction at the high-pH end changing to a pH-independent region at lower pH. The change in absorbance at 235 nm of the relatively stable (E)-trifluoroethanediazoate as a function of pH gives a good fit to a simple titration curve for a monofunctional acid with pK(a) = 7.00 +/- 0.04 that is in good agreement with the kinetic pK(a) determined from the pH-rate profile between pH 4 and 12. The plot of log k(1), the pH-independent rate constant, against sigma* for the primary (E)-alkanediazoates, and (E)-methane- and (E)-2-butanediazoates previously reported, yields a common line with a slope rho* = -4.4. Of the 1-butanol formed from the decomposition of (E)-1-butanediazoate in D2O at pH(obs),b, = 10.50, 6% contains deuterium, while, of the 1-butanol formed in the presence of 1 M NaOD in D2O, 25% contains deuterium. A total of 96% of the iodotrifluoroethane formed upon decomposition of (E)-trifluoroethanediazoate in a D2O solution that is 1 M in NaI contains at least a single deuterium atom. These data combined with small negative values of Delta S#, normal solvent deuterium isotope effects, and the decreases in k(1) of between 500- and 1000-fold on change of solvent to ethanol are consistent with a mechanism that involves the rate-limiting unassisted heterolytic bond fission of the diazoic acid to yield the diazonium ion.