The addition of P to a Ni/SiO2 catalyst greatly changes the isobutane dehydrogenation performance. The Ni:P ratio is an important influencing factor for a Ni-P/SiO2 catalyst because of the formation of different Ni-P compounds. When the Ni:P ratio equals 1:1, a Ni-P/SiO2 catalyst with Ni2P formed on the surface exhibits an optimum dehydrogenation performance (an isobutane conversion of 22% and an isobutene selectivity of 81.3%). The in situ Fourier transform infrared characterization was performed to determine the adsorption mode of isobutane. Isobutane is adsorbed onto the Ni-P surface with one H atom in a methyl group, and the intermediate tends to form isobutene rather than C-C bond scission. Moreover, the decreased adsorption energy of isobutene on the Ni2P surface inhibits its further reaction. The deactivation of a Ni-P/SiO2 catalyst is mainly caused by the phase transformation from Ni2P to Ni12P5 and coke deposition. Finally, some general rules are summarized to get a deep understanding of the second component added to a Ni/SiO2 catalyst.