Although coke formation is one of the most important problems that the steam cracking process still faces, the properties of the formed coke have hardly been studied over the past decade. For some of them, orders of magnitude differences have been reported. Therefore, state-of-the-art measurement techniques have been used on industrially relevant samples obtained at laboratory and industrial scale for ethane and naphtha cracking. The feedstock type does play an important role, reflected in, for example, factor 2 differences in BET surface areas for ethane and naphtha coke samples. Molar C/H ratios from 26 up to 59 are obtained by CHNS/O analysis, depending on the age of the coke and the proximity of the metal coil. The measured thermal conductivity values for ethane and naphtha coke, that is, 9.6 and 8.7 W m(-1) K-1 at 1273 K, respectively, are approximately twice and three times the values reported by Dente and co-workers. A sensitivity analysis on the influence of these properties on the predicted run length of an industrial furnace gives huge differences, that is, from 2 weeks up to 212 days for naphtha furnace operation and 20 up to 227 days for ethane operation. This clearly indicates the importance and the need for more accurate assessment of coke properties to develop more predictive models for run length simulations.