Combustion of hydrazinium nitroformate (HNF), N2H5C(NO2)3, has been modeled and the resultscompared with experimental observations including steady regression rate (pressure, initial temperature, and radiant flux sensitivities), surface temperature, and linear frequency response to radiation. The underlying philosophy of the approach is to compare HNF combustion with a wide range of experimental conditions as much as possible. The results indicate that HNF condensed-phase decomposition is overall slightly exothermic (increasing with pressure) with activation energy similar to that for proton transfer between hydrazinium and nitroformate ions. The gas-phase flame is highly exothermic (also slightly increasing with pressure) with low-activation energy, suggesting the gas-phase process is more like a lowenergy barrier (low Eg), chain-carrier process than a high-energy thermal process, similar to what has been suggested for nitrocellulose-nitroglycerine double-based propellant and for cyclo-tetramethylene-tetranitramine. For both of these materials, and now for HNF, the low-Eg model has been shown to be in good agreement with experimental data for both steady and unsteady burning rate. The model also demonstrates the ability to capture an importants manifestation of nonlinear combustion. The prediction of a stronginitial pressurization spike in a low L* (high dP/dt) end-burning (no cross-flow) motor with HNF propellant confirms our recent hypothesis that initial pressurization spikes may be due to nonlinear dynamic combustion in addition to (or instead of) erosive burning.