The structural changes accompanying the electronic excitation of the antiaromatic biphenylene molecule were studied by semiempirical quantum chemical calculations, using the AM1 hamiltonian with configuration interaction (S 1 , S 2 , T 1 , T 2 , X + states). The largest bond length differences of the molecular states investigated were observed within the central four-membered ring. Potential curves of the CC bonds which connect the two benzene rings have been calculated for all molecular states investigated. For the S 0 and S 1 states, potential hypersurfaces, using the two different bond lengths of the central four-membered cyclobutadiene subsystem as variables, were presented. Furthermore, the geometry of 1,3-cyclobutadiene in different states (S 0 , S 1 , T 1 , X + states) was investigated with the AM1-CI hamiltonian and compared with recent 6-31G * MC-SCF ab initio results.