The influence of temperature, residence time, concentration level of reactants and products, polymer type, and composition of the polymer mixture on the product spectra obtained from pyrolysis of polyethene (PE) and polypopene (PP) was determined. In our study the temperature was varied between 650 and 850 °C, whereas residence times between 0.1 and 1 s were used. Thermodynamic calculations revealed that aromatics and methane are formed as the main products under conditions pertaining to chemical equilibrium. The ratio between these compounds is governed by the C/H ratio in the original polymer. Experiments were performed using a tubular reactor. The main products of the pyrolysis of PE and PP at 750 °C and a residence time of 1 s are ethene (respectively 45 and 19 wt %), propene (24 and 45 wt %), and butene (23 and 27 wt %). At higher reactor temperatures the yields of ethene and methane increase, while the yields of both propene and butene decrease. The influences of the residence time, product concentration, polymer type, and composition of the polymer mixture on the product distribution are negligible compared to the influence of temperature.