A combined experimental and kinetic modeling study is presented to improve the understanding of the formation of polycyclic aromatic hydrocarbons at pyrolysis conditions. The copyrolysis of cyclopentadiene (CPD) and ethene was studied in a continuous flow tubular reactor at a pressure of 0.17 MPa and a dilution of 1 mol CPD/1 mol ethene/10 mol N-2. The temperature was varied from 873 to 1163 K, resulting in cyclopentadiene conversions between 1 and 92%. Using an automated reaction network generator, RMG, we present an elementary step kinetic model for CPD pyrolysis that accurately predicts the initial formation of aromatic products. The model is able to reproduce the product yields measured during the pyrolysis of pure cyclopentadiene and the copyrolysis of cyclopentadiene and ethene. The addition of ethene as coreactant increases the benzene and toluene selectivity. In the absence of ethene, benzene formation is initiated by addition of a cyclopentadienyl radical to cyclopentadiene, following a complicated series of isomerizations and loss of a butadienyl radical. In the presence of ethene, the main pathway for the formation of benzene + CH3 shifts to ethene + cyclopentadiene. Toluene formation is initiated by vinyl radical addition to cyclopentadiene. Without the addition of ethene, vinyl radicals are mainly formed by hydrogen radical addition to ethyne. When ethene is added as coreactant, vinyl radical production happens via hydrogen abstraction from ethene.