Abstract: A hydrocarbon feedstock derived from crude oil or other sources is subjected continuously to thermal cracking under pressure and in the presence of hydrogen in order to convert the feedstock to products of lower molecular weight containing high proportions of olefinic components. In a first processing step, a catalytic hydrogenating pretreatment is performed at a temperature within the range of 300.degree. C. to 600.degree. C. and under a pressure within the range of 12 to 85 bars. In a second step, a thermal cracking treatment is performed under a pressure within the range of 10 to 70 bars with residence times of less than 0.5 second and at a temperature within the range of 625.degree. C. to 1,000.degree. C., the quantity of hydrogen employed being such that the molar concentration in the effluents is at least equal to 20%.