Exact knowledge of the local temperature during the compression of air/fuel-mixtures in engines is crucial for evaluating modeling results of the ignition conditions and the flame development after ignition. Especially in ultra-lean burning modern engines using stratified load and exhaust gas recirculation inhomogeneous mixing takes place and causes inhomogeneous temperature distributions. Thus two-dimensional temperature imaging techniques have to be applied to provide the necessary information for model calculations. Temperature measurements in the compression stroke both before and after ignition have been performed in specially designed optically accessible engines using Rayleigh scattering 1. In more production-like engines, however, due to background scattering the use of Rayleigh scattering for temperature measurements is very limited. It has been demonstrated before2,3 that laser induced Fluorescence (LIF) of ketones like acetone and 3-pentanone is susceptible to temperature, depending on the excitation wavelength. This is mainly due to a temperature-dependent shift of the common absorption feature which ranges from 220 nm to 340 nm, peaking near 280 nm at room temperature. The resulting fluorescence occurs between 330 nm and 550 nm and is approximately independent of the excitation wavelength for 3-pentanone. Due to evaporation properties similar to those of the non-fluorescent model fuel iso-octane, 3-pentanone is an ideal marker used for mapping fuel distributions in SI engines, particularly when excited close to its absorption maximum where the fluorescence signal is nearly independent of temperature. When excited in the wings of the absorption spectrum, however, the signal is strongly dependent on temperature. This suggests the feasibility to quantitatively measure temperatures, as first demonstrated for static conditions by Grossmann et al.