In this paper,the simultaneous measurement techniques of SO2 and SO3 at high temperatures based on mid infrared absorption spectroscopy using a quantum cascade laser (QCL) spectrometer is proposed.SO3 is produced by the heterogeneous catalytic oxidation of SO2 with S 101 intermediate temperature vanadium catalysts and the SO2/SO3 concentrations and the conversion rates are measured during the chemical reaction at pressures of 3～20 kPa and temperatures of 550～1000K.The SO2 spectra in the ν3 band and the SO3 spectra in the ν3+ν4 band near 7.16 m at high temperatures are well resolved in the strongly overlapping spectra by modifying the SO2 and SO3 high temperature spectral models based on the room temperature spectra in the HITRAN database.A SO2/SO3 separation unit was designed to extract and measure the remaining SO2 after conversion at room temperature,with the results comparing well with the optically measured concentrations in the high temperature catalytic reactions.The accuracy of the SO2 concentration obtained from the synchronous measurements is verified by comparison.No SO3 is detected at temperatures lower than 590K but obvious adsorption of SO2 on the catalyst surface is observed and the adsorption effect increased when increasing pressures.SO3 is first detected at temperatures higher than 590K with the SO2 conversion rate increasing to 92% at 800K and then decreasing to less than 3% at 1000K.The reaction conversions show a logarithmic growth with increasing pressures at each temperature.