Dinuclear platinum(II) complexes [Pt-2(mu-pz)(2)(bpym)(2)](2+) (1; pz = pyrazolate and bpym = 2,2'-bipyrimidine) and [Pt2(p-pyt)2(ppy)72] (2; pyt = pyridine-2-thiolate and Hppy = 2-phenylpyridine) were theoretically investigated with density functional theory (DFT) to clarify the reasons why the phosphorescence of 1 is not observed in the acetonitrile (CH3CN) solution at room temperature (RT) but observed in the solid state at RT and why the phosphorescence of 2 is observed in both the CH3CN solution and the solid state at RT. The S-1 and T-1 states of 1 in the CH3CN solution are assigned as a metal metal-to-ligand charge-transfer (MMLCT) excited state. Their geometries are C-2, symmetrical, in which spin orbit interaction between the SI and T-1 excited states is absent because the direct product of irreducible representations of the singly occupied molecular orbitals (SOMOs) of these excited states and the orbital angular momentum (/) operator involved in the Hamiltonian for spin orbit interaction does not belong to the a(1) representation. As a result, the SI T-1 intersystem crossing hardly occurs, leading to the absence of T-1 So phosphorescence in the CH3CN solution at RT. In the solid state, the geometry of the Si state does not reach the global minimum but stays in the Crsymmetrical local minimum. This S-1 excited state is assigned as a mixture of the ligand-centered pi-pi* excited state and the metal-to-ligand charge-transfer excited state. Spin orbit interaction between the SI and T-1 excited states operates to induce the S-1 T-1 intersystem crossing because the direct product of the irreducible representations of the SOMOs of these excited states and the operator belongs to the "a" representation. As a result, T-1 So phosphorescence occurs in the solid state. In 2, the S-1 and T-1 excited states are assigned as the MMLCT excited state. Their geometries are C-2-symmetrical in both the CH3CN solution and the solid state, in which spin orbit interaction between the Si and T-1 states operates to induce the Si T-1 intersystem crossing because the direct product of the irreducible representations of the SOMOs and the I operator belongs to the "a" representation. Thus, T-1 So phosphorescence occurs in both the CH3CN solution and the solid state at RI, unlike 1.