The combined use of nitric and formic acids, in lieu of formic acid alone, to reduce H2 emissions during the treatment of high-level radioactive waste sludge was investigated. The H2 generation can be mitigated substantially by substituting a fraction of formic acid with nitric acid as the required acid source, and then using formic acid as the required reductant source. The peak H2 generation rate was reduced by more than a factor of 2, and a more gradual rise in the H2 evolution resulted. However, the addition of mercury to the sludge increased the evolution of H2 as did increasing the amount of nitric acid used and the rate of addition of the formic acid source. Overall, these results provided clear insight into what controlled the evolution of H2 from high-level waste sludge and a means of mitigating it.