This work was focused on studying selected cyclic and acyclic hydrocarbons reactivity (assessed according to conversion) during pyrolysis using the laboratory pyrolysis gas chromatography. The pyrolysis of individual hydrocarbon and copyrolysis of aliquot part of the tested compound in its mixture with primary gasoline was investigated. The behaviour of tested compounds was studied on the basis of laboratory experiments carried out at standard temperature (815 °C) and short residence time in the hot-zone of the reactor (carrier gas velocity was 100 ml min^–1). The conversion of compounds that are less reactive individually increased during copyrolysis thanks to higher amounts of small active radicals in reaction mixture. The reactivity of hydrocarbons increased with increasing length of chain, number of carbon atoms in cycle, number and length of substituents on the ring and decreased with increasing number of cycles in the hydrocarbon molecule. The effect of active radicals (developed from decomposition of gasoline) on reactivity of unsaturated hydrocarbons was minimal.