The effect of laser heating on the local concentration of a white Ba(NO<sub>3</sub>)<sub>2</sub> or BaO<sub>2</sub> sample in a stationary in situ Raman cell, as determined by Raman spectroscopy, was addressed in this study. The decomposition of Ba(NO<sub>3</sub>)<sub>2</sub> and crystalline BaO<sub>2</sub>, both supported on MgO, was followed as a function of the temperature of the cell with the use of Raman spectroscopy and product analysis by gas chromatography. At a laser power level of 25 mW and with the sample at 580°C, the decomposition curves were offset by about 3°C, rather than by a temperature difference of more than 15°C that might be expected from laser heating. Diffusion of NO<sub>3</sub>- and O<sub>2</sub><sup>2</sup>- ions from the surrounding region into the 100 mu m-size spot heated by the laser apparently compensates for the loss of materials at the higher temperature. As a consequence, for in situ catalytic studies, the concentrations determined by Raman spectroscopy may be more representative of the sample than one might expect from the local temperature.

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