The rate constant for the reaction of the hydrated electron with thiosulphate has been measured in the absence and presence of sodium sulphate. The bimolecular rate constant obtained by dividing the observed pseudo first order rate constant by the concentration of Na 2S 2O 3 did not change significantly when a fraction of the Na 2S 2O 3 was replaced by Na 2SO 4. It is concluded that both salts have the same physical effects on the rate constant. Neither salt is fully dissociated at the concentrations employed so that the solutions contained S 2O 2- 3 and NaS 2O - 3. Rate constants for the reaction of e - aq with each ion could not be calculated, but a value of k(e - aq+S 2O 2- 3) of ca. 5 X 8 dm 3 mol -1 s -1 at zero ionic strength was estimated from a short extrapolation from 1.07 x 10 -2 mol dm -3 Na 2S 2O 3 for which k=(1.82 ± 0.02) x 10 8 dm 3 mol -1 s -1. This value is different from previously published values which vary by a factor of 24. For ca. 0.1 mol dm -3 solutions of S 2O 2- 3 the end-of-pulse yield of e - aq was found to be the same as for 10 -3 mol dm -3 H + aq, indicating that S 2O 2- 3 does not scavenge presolvated electrons at that concentration. Such solutions, therefore, could not inhibit muonium formation in they provide a background against which to test models of muonium formation.