If the processability of heavy or residual oil is assessed in terms of conversion to distillate oils, the tendency to form carbonaceous (coke) deposits in the reactor would impose limits on the operating conditions, since any significant coke accumulation on a continuing basis would be prohibitive. Experimentally, one can evaluate the feedstock coking propensity in a bench-scale reactor for a set of typical hydroprocessing conditions by varying the temperature near the threshold of coking. For a number of different feedstocks examined by this method, an empirical function can be found correlating the coking propensity with some of the characteristic properties determined by routine analyses. The present approach used a combination of physical and chemical properties including density and contents of Conradson Carbon residue, pentane and toluene in-solubles, fraction of high-boiling pitch, and carbon, hydrogen, nitrogen, sulfur and ash. Feedstocks were chosen to cover a wide but practical range of properties to establish their impact on the coking propensity. Although these relationships apply to thermal hydroprocessing, the role of catalysts is also indicated.