A method is provided for the measurement of parameters of a gas present in a gas turbine combustion chamber. The method includes tuning a laser to a range containing the absorption lines of species to be analyzed in the gas, and directing the laser light through the combustion chamber and detecting laser light reflected off boundary walls of the combustion chamber. In order to analyze the absorption spectrum measured at high temperatures and pressures, a signature recognition algorithm is applied to the spectrum. The measured absorption spectrum is cross-correlated with a calibration absorption model spectrum for the absorption lines at several temperatures, pressures, and concentrations generated prior to the measurement. Values for pressure, temperature, and concentrations of selected species in the gas are determined simultaneously allowing direct control of the combustion chamber process. An apparatus for carrying out the method is also provided.