A high-boiling product from petroleum vacuum distillate hydrocracking was used as a feedstock for pyrolysis to light alkenes. The high boiling product was distilled to yield five fractions. Pyrolysis of these fractions was performed in a laboratory pyrolysis apparatus. The fractions pyrolysed were analyzed by a procedure consisting of their separation by liquid adsorption chromatography and subsequent characterization of the chromatographic fractions obtained by elemental analysis, vapour pressure osmometry, H-1 and C-13 NMR spectrometry and mass spectrometry. In this way the relation between the chemical structure and composition of the pyrolysed fractions and the yield of light alkenes was obtained. It was found that the fraction of the high-boiling product boiling above 486 degrees C was not a suitable pyrolysis feedstock even if its BMC index was good; this fraction had a low alkane content.