A process and apparatus for determining the hydrogen-to-carbon ratio of the liquid fraction of a pyrolysis product by determining the hydrogen-to-carbon ratio of the pyrolysis product gas fraction and subtracting the thus determined value from the determined hydrogen-to-carbon of the hydrocarbon feed to the pyrolysis furnace. The determined value for the hydrogen-to-carbon ratio of the liquid fraction is used as a measure of cracking severity and in the control of the pyrolysis furnace in response to differences between the desired and actual hydrogen-to-carbon ratio of the liquid fraction.