Phase evolution during the hydrogen-assisted disproportionation of Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 alloy was investigated mainly by using a magnetic balance-type thermomagnetic analyser (TMA). In order to avoid any undesirable phase change in the course of heating for TMA, a swift TMA technique with very high heating rate (around 2 min to reach 800 ^oC from room temperature) was adopted. The hydrided Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 alloy started to be disproportionated in hydrogen from around 600 ^oC, and the alloy after the early disproportionation (from 600 to 660 ^oC) has been partially disproportionated. The partially disproportionated alloy consisted of a mixture of NdH_x, Fe₃B, α-Fe, and the remaining undisproportionated Nd₂Fe₁₄BHx-phase. During the subsequent heating to 800 ^oC in hydrogen, two additional phases of Fe₂₃B₆ and Fe₂B were formed, and the material consisted of a mixture of NdH_x, Fe₂₃B₆, Fe₃B, Fe₂B, and α-Fe phases. During the subsequent isothermal holding at 800 ^oC for 1 hour, the phase constitution was further changed, and one additional unknown magnetic phase was formed. Eventually, the fully disproportionated Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 alloy consisted of NdH_x, Fe₂₃B₆, Fe₃B, Fe₂B, α-Fe, and one additional unknown magnetic phase.