The mechanism of the photochemical synthesis of pentalene was investigated with the semiempirical method SINDO1. The potential hypersurfaces of the relevant low-lying singlet and triplet states were studied, and intermediates and transition states were optimized with limited CI. Starting with 5,6-diazaazulene as a precursor a reaction pathway to pentalene is proposed. In the first reaction tricyclo[5.3.0.0(3.6)]-4,5-diazadeca-1,4,7,9-tetraene is formed via internal photochemical ring closure. This tricyclic system loses N-2 in a subsequent photochemical reaction yielding pentalene. The photochemical elimination of the N-2 molecule is adiabatic and simultaneous in either the T-1 or the S-1 state. Pentalene is then found in the first triplet or first excited singlet state.