A technique for rapid in situ measurement of the fuel dilution of oil in a diesel engine is presented. Fuel dilution can occur when advanced in-cylinder fuel injection techniques are employed for the purpose of producing rich exhaust for lean NOx trap catalyst regeneration. Laser-induced fluorescence (LIF) spectroscopy is used to monitor the oil in a Mercedes 1.7-liter engine operated on a dynamometer platform. A fluorescent dye suitable for use in diesel fuel and oil systems is added to the engine fuel. The LIF spectra are monitored to detect the growth of the dye signal relative to the background oil fluorescence; fuel mass concentration is quantified based on a known sample set. The technique was implemented with fiber optic probes which can be inserted at various points in the engine oil system. A low cost 532-nm laser diode was used for excitation. Measurements of fuel dilution of oil are presented for various in-cylinder injection strategies for rich operation of the diesel engine. Measurements with the LIF technique are compared with standard gas chromatography measurements. Rates of fuel dilution increase for all strategies relative to normal lean operation, and higher fuel dilution rates are observed when extra fuel injection occurs later in the combustion cycle when fuel penetration into the cylinder wall oil film is more likely.