The gas chromatographic measurement of the ortho (=triplet nuclear spin)/para (=singlet nuclear spin) ratio of hydrogen molecules generated by gamma-irradiation of alkanes revealed that the ortho/para ratio depends on the physical phase of alkanes during the irradiation. The ortho/para ratio for liquid alkanes was equal to the high-temperature limit of 3, which implies that the rate of hydrogen atom abstraction from alkanes by hydrogen atoms, the main mechanism of the formation of hydrogen molecules, does not depend on the nuclear spin states of the abstracting and abstracted hydrogen atoms, The ortho/para ratio for solid alkanes was about 2.6, which indicates that hydrogen molecules are trapped for more than a few second adjacent to the alkyl radicals generated in pair with the hydrogen molecules, during which the lattice vibration of the alkyl radicals induces the simultaneous transition of the rotational and the nuclear spin states of the hydrogen molecules. A quantum-mechanical theory of the ortho-para transition has been given.