The electronic structure of magnesiumphthalocyanine (MgPc) has been investigated on single crystals grown from the vapor phase. MgPc is found to crystallize in the space group of P2₁/a (monoclinic): Z=2, a=18.971(2), b=4.916(2), c=14.401(2) Å, β=119.890(9). The MgPc molecule is not flat, but the Mg atom is projected out of the plane of the four central nitrogen atoms by about 0.45 Å, forming a pyramidal structure (c₁ symmetry). The present molecular deformation has, however, no significant influence on the optical absorption according to the MO spectroscopic calculations. On the other hand, the molecular distortion induces a drastic change in the direction of the transition dipoles, so that the interactions between transition dipoles in the solid state are strikingly different from those of ordinary phthalocyanines and the polarized reflection spectra look therefore quite unique.