When applying the differential optical absorption spectroscopy (DOAS) technique for in-situ monitoring of flue-gas emissions, two main problems may occur in comparison to the atmospheric monitoring DOAS application. The first problem is due to the high and variable temperatures of flue gases, which significantly affects the magnitude of the differential absorption cross-sections. The second problem is caused by the limited choice of optical path- lengths, causing non-linearity effects due to large gas absorption. Measurements of the differential absorption cross-sections for NO, NO2, and SO2 have therefore been performed in a pyrex-glass cell contained in a heat-pipe, at temperatures between 20 and 400 degree(s)C, in the wavelength range of 205 to 440 nm. We also have performed measurements of the linear regions of the technique for measurements of SO2, NO2, and NO. The linear regions were shown to be 1 - 3200 mgm-2 for NO2, 0 - 2000 mgm-2 for SO2, and 0 - 120 mgm-2 for NO, in the spectral resolution range between 0.2 to 0.95 nm. The differential absorption cross-sections of NO2 and SO2 are strongly temperature dependent causing considerable errors in evaluated concentrations when using the DOAS technique. The relative errors due to temperature were of the order of 70% for SO2 and NO2 and of the order of 15% for NO at 400 degree(s)C.