The polarizations of the transitions to the lowest nπ* and the two lowest ππ* transitions of phthalaldehyde have been measured relative to the phosphorescence by the method of photoselection. The polarization of transitions to the lowest 10-methylene anthrone nπ* state and the 1La and 1Lb ππ* states, which are split by exciton interaction, has also been determined. Molecular orbital calculations, spin-orbit coupling, and considerations of the experimental results indicate that the T1← S0 transition of 10-methylene anthrone is polarized along the carbonyl carbon-oxygen bond (Z axis). Application of Platt's spectroscopic moment theory to the 10-methylene anthrone system, considered as two weakly interacting benzene units perturbed by formyl and vinyl substituents, does not account for the observed polarization in the ππ* region for a Z-polarized phosphorescence transition moment. The polarization curve in the ππ* region is explained on the basis of perturbed 1La and 1Lb benzene states split by inter-ring exciton interaction. The nπ* transition is believed to be polarized in the molecular plane and perpendicular to the carbonyl axis; that is, along the Y axis. An alternation in polarization in the nπ* region is observed. Indirect evidence is presented which indicates that the T1← S0 transition in phthalaldehyde is primarily polarized along the Z axis. If this transition is localized in the formyl groups, a small out-of-plane component of the transition moment may be present since the formyl groups are not expected to be in the molecular plane. The higher ππ* transition in phthalaldehyde is polarized parallel to that of the phosphorescence and is assigned to the La1←A1 benzene analog transition. The lower ππ* transition is perpendicular and is assigned to the Lb1←A1 transition on the basis of Platt's spectroscopic moment theory. The polarization in the phthalaldehyde nπ* region is uncertain but appears to be polarized in the molecular plane and approximately parallel to the carbonyl carbon-oxygen bond; that is, along the Z axis. The lifetime of the phthalaldehyde phosphorescence was found to be 3 msec.