Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac2, Th 2, Pa2, and U2. Scalar relativistic effects and spin-orbit coupling have been included in the calculations. In the Ac 2 and Th2 diatoms the atomic 6d, 7s, and 7p orbitals are the significant contributors to the bond, while for the two heavier diatoms, the 5f orbitals become increasingly important. Ac2 is characterized by a double bond with a 3Σg-(0g+) ground state, a bond distance of 3.64. Å, and a bond energy of 1.19 eV. Th2 has quadruple bond character with a 3D g(1g) ground state. The bond distance is 2.76 Å and the bond energy (D0) 3.28 eV. Pa2 is characterized by a quintuple bond with a 3Σg- (Og +) ground state. The bond distance is 2.37 Å and the bond energy 4.00 eV. The uranium diatom has also a quintuple bond with a 7Og (8g) ground state, a bond distance of 2.43 Å, and a bond energy of 1.15 eV. It is concluded that the strongest bound actinide diatom is Pa2, characterized by a well-developed quintuple bond.