DFT-B3LYP studies have been used to understand the realm of [2 + 2] photocycloaddition reactions that lead to the formation of four-membered cyclobutane derivatives. Regioselectivity in these reactions has been explored for the typical system 6-amino-2-(3'-thienoyl)-1,4-benzoquinone and ethylene. The results favor initial attack of ethylene on quinonoid carbon C5 rather than any other position.