Calculations are reported on the structure, transition energies, and oscillator strengths of zinc phthalocyanine using a combination of the AM1 method and a version of the CNDO/S method, respectively. The origin of the experimental Q- and B-absorption bands has been explored in terms of the electronic transitions between occupied and virtual orbitals. The change in position of the electron densities in the frontier HOMO and LUMO molecular orbitals has been used to rationalize the effect of substituents on the experimental absorption maximum. Electron donors in the phenyl rings facilitate electron transfer from the HOMO to the LUMO and result in large bathochromic shifts.