Coal and gas outbursts are a violent release of energy in part driven by rapidly desorbing gas from the fragmenting coal. We present a coupled two-phase model of coal and gas outbursts to define the timing, rate and magnitude of gas desorption and its contribution to the resulting energetics. The model involves a fragmenting ejection of the outburst from an overpressurized coal that retreats omnidirectionally from a point and develops a deepening crater. This model is applied to represents both experiments and in situ observations. These results indicate that the outburst is initially driven by free gas before desorbing gas rate exceeds this free gas liberation rate early into the outburst (at ∼17 s in our model). The cumulative mass of desorbed gas only exceeds the free gas later into the event at approximately double this duration (at ∼29 s in our model). During the outburst, ∼55 % of the expansion energy is contributed by desorbing gas. Using the initial gas emission rates for both non-tectonic and tectonic coals defines a power-law relationship between the initial gas desorption rate and the desorption gas contribution (∼14 %–92 %), indicating that the desorbing gas plays a decisive role in outburst development. Furthermore, taking the gas emission model as a boundary conditions for numerical simulations, the gas pressure potential energy (GPPE) released in the first millisecond at the maximum gas emission rate is derived to characterize its effects on the dynamic characteristics of the outburst two-phase flow. The maximum energy release intensity considering gas desorption is ∼5 times that without gas desorption for non-tectonic coal. For tectonic (mylonitized) coals the energy release is a further ∼4 times greater than that of non-tectonic coals. This paper presents a novel quantitative study defining the role of gas desorption in outbursts and contributes to the understanding of causal mechanisms and precursory phenomena preceding catastrophic outbursts.