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The thermal decomposition of nitrogen oxides (NO_x) on phosphotungstic acid (H₃PW₁₂O₄₀·6H₂O or HPW) by two different heating methods is compared. Infra-red (IR) and X-ray diffraction (XRD) measurements are conducted to investigate the decomposition mechanism. Both heating methods, i.e. heating from 30 °C to 450 °C at a rate of 150 °C/min (“rapid heating”) and heating at a constant temperature of 450 °C (“constant-temperature heating”) lead to an actual, considerably high heating rate. Compared with rapid heating, however, constant-temperature heating results in enhanced N₂ conversion (21.8%). Furthermore, the catalyst can be reused after decomposition at constant-temperature heating, while its performance quickly degrades after decomposition via rapid heating.