In situ simultaneous measurements of gas temperature and gas composition are of great interest in combustion research and give useful information about conditions, chemical reactions and gas mixing in many industrial processes. An optically based technique is beneficial because it is non-intrusive, accurate, fast and can be performed in situ for various extremely hard conditions. In humid and hot gas flows UV technique is more sensitive than FTIR one for fast gas concentration measurements of NO and SO2 and gives a great opportunity for simultaneous measurements of O2 concentration. Analysis of the fine structure of the UV absorption bands of, for example, NO, SO2 or O2 allows also to determine a value of the gas temperature. Absorption cross sections of CO2, H2O and SO2 measured using Risø DTU’s hot gas cell facility at elevated temperatures up to 1500°C are reported. Design of a new developed 9-m long water-cooled fiber-optic probe with removable optical head suitable for fast IR/UV local gas absorption/emission measurements is described. The probe performance was successfully tested in several trial measurements on full scale multi-fuel fired boiler. A concept of fast time/spectralresolved measurements has been used in measurements on a large ship engine based on IR and UV broad band spectroscopy.