This experiment emphasizes the first example of two-phase sequential Rietveld refinements throughout a solid/gas chemical reaction monitored by Neutron Powder Diffraction (NPD) at high temperature. The reduction of the n = 1 Ruddlesden-Popper (RP) oxide Sr2MnO4 heated under a flow of 5% H-2-He has been investigated throughout two heating/cooling cycles involving isothermal heating at 500 and 550 degrees C. Oxygen loss proceeds above T similar to 470 degrees C and increases with temperature and time. When the oxygen deintercalated from the "MnO2" equatorial layers of the structure results in the Sr2MnO3,69(2) composition, the RP phase undergoes a first order I4/mmm -> P2(1)/c, tetragonal to monoclinic phase transition as observed from time-resolved in situ NPD. The phase transition proceeds at 500 degrees C but is incomplete; the weight ratio of the P2(1)/c phase reaches similar to 41% after 130 min of isothermal heating. The fraction of the monoclinic phase increases with increasing temperature and the phase transition is complete after 80 min of isothermal heating at 550 degrees C. The composition of the reduced material refined to Sr2MnO3.55(1) and does not vary on extended heating at 550 degrees C and subsequent cooling to room temperature (RT). The symmetry of Sr2MnO3.55(1) is monoclinic at 550 degrees C and therefore consistent with the RT structure determined previously for the Sr2MnO3.64 composition obtained from ex situ reduction. Consequently, the stresses due to phase changes on heating/cooling in reducing atmosphere may be minimized. The rate constants for the reduction of Sr2MnO4.00 determined from the evolution of weight ratio of the tetragonal and monoclinic phase in the time-resolved isothermal NPD data collected on the isotherms at 500 and 550 degrees C are k(500) = 0.110 x 10(-2) and k(550) = 0.516 x 10(-2) min(-1) giving an activation energy of similar to 163 kJ mol(-1) for the oxygen deintercalation reaction.