A method for assaying a flowing gas mixture for SO2(g) and SO3(g) is described. The proof-of-concept for this method presented here uses the quantitative reaction Of SO3(g) with CaC2O4(s) at 325 degrees C to produce CO2(g). The gas mixture is passed first through a plug of CaC2O4 and then through a series of two optical cells. SO2(g) is measured directly at one cell by UV spectrometry, and SO3(g) is measured indirectly as CO2(g) at the second cell by IR spectrometry. Good correlation coefficients were found for both analytes. Potential interference by SO2 on the IR response Of CO2 can be neglected within a 1% error margin-except when the ratio CO2/SO2 is very small. A summary of key analytical methods for the analysis of sulfur oxides is given.