Recent theoretical investigations of the radical/pi-bond addition between single-ring aromatic hydrocarbons highlight the importance of this category of reactions for the formation of PAR intermediates and soot. The present investigation extends the theory of the radical/pi-bond addition reactions to the o-benzyne + cyclic C-5 hydrocarbons systems. The calculations, performed using the uB3LYP/6-311+G(d,p) method, have addressed the possible role of the reaction between o-benzyne and cyclopentadiene in the formation of indene through the fragmentation of the bicyclo intermediate benzonorbornadiene. The complex potential energy surface for the reaction between o-benzyne and cyclopentadienyl radical was also investigated. In this case, the formation of the bicyclo benzonorbornadienyl radical and its subsequent fragmentation to indenyl radical and acetylene is not the main reaction pathway, although it could be relevant at relatively high temperatures. At lower temperatures, the isomerization reactions, which lead to the formation dominant.