Future engine emission legislation regulates soot from Diesel engines strictly and requires improvements in engine calibration, fast response sensor equipment and exhaust gas aftertreatment systems. The in-cylinder phenomena of soot formation and oxidation can be analysed using a pyrometer with optical access to the combustion chamber. The pyrometer collects the radiation of soot particles during diffusion combustion, and allows the calculation of soot temperature and a proportional value for the in-cylinder soot density (KL).A four-cylinder heavy-duty Diesel engine was equipped in all cylinders with prototype pyrometers and state of the art pressure transducers. The cylinder specific data was recorded crank angle-resolved for a set of steady-state and transient operating conditions, as well as exhaust gas recirculation (EGR) addition and over a wide range of soot emissions. A continuously operating gas sampling photo acoustic soot sensor was used for measuring the exhaust gas soot concentration.A correlation of cylinder specific pyrometer measurements and corresponding tail pipe soot concentrations for a set of operating conditions was developed including variability of start of injection, engine load and speed, injection pressure and EGR. The correlation matches the in-cylinder soot concentration at the end of the combustion well with exhaust pipe soot emissions. In transient operation, the predicted soot emissions showed immediate response on changing engine load, speed and their combination whereas the soot concentration sampled in the exhaust stream was delayed due to gas mixing. The agreement between the two measurement methods was good. Furthermore a qualitative analogy between soot temperature and cylinder specific EGR rate was found.