Rising greenhouse gas emissions caused by the transport sector are one factor that contributes to global warming. Consequently, the EU has decided on new regulations concerning the emissions of automobiles and on the introduction of more realistic test procedures for the admission of new vehicles. The automotive industry now faces the challenge to develop more efficient internal combustion engines. The use of alternative fuels like compressed natural gas is one possible option to reduce the CO 2 emissions of combustion engines. To achieve this goal, new measurement techniques, which allow the analysis of mixture formation prior to combustion, are necessary. In this thesis the development of an optical measurement procedure for time resolved quantification of the λ-value in combustion engines is presented. The procedure is based on the measurement of the broadband transmittance of the ν 3 -absorpion line of methane. Based on absorption cross sections taken from the HITRAN database the measured transmittance can be linked to a density via the Beer-Lambert's Law. For this purpose the spectral influences of the measurement system as well as pressure and temperature are taken into account. Utilizing a 2-filter-method, which uses the transmission signals of different parts of the methane spectrum, density and temperature inside the measurement volume can be determined simultaneously. A spark plug probe allows measurements of the λ-value at the location of the ignition. Additionally, an inlet manifold probe is developed in this thesis, which allows the analysis of the mixture formation at the location of the fuel injection. The measurement system and the newly developed algorithm for the analysis of the measurement data are tested with pressure cell experiments. Remaining differences between measurement and calculation are eliminated using a calibration procedure. Through extensive measurements at a rapid compression machine and two different test engines the performance of the measurement system is evaluated and accuracy and precision are determined. The results show that the measurement system developed in this thesis allows for an accurate, time resolved analysis of mixture formation in gas engines.