Pyrolysis of topped crudes has been carried out at constant heating rates. The coke yield y versus peak pyrolysis temperature T has been found to be approximated by a Robertson curve, y = a/[1 + exp(b - akT)], where a, b and k are constants. The logarithm of the constant a, which corresponds to the ultimate coke yield, decreases linearly with the reciprocal of the aromaticity of the feedstock and is independent of heating rate. Both In b and In k increase linearly with the reciprocal of the aromaticity of the feedstock. The constant k decreases linearly with the logarithm of heating rate. The maximum coking rate a(2)k/4 (wt% deg(-1)) decreases with the logarithm of heating rate. The temperature b/ak (degrees C) indicating the maximum coking rate decreases with the aromaticity and rises with heating rate. The temperature range 4/ak (deg) for coke formation increases with both the aromaticity and heating rate.