The kinetic features of the process of 4-phenylphthalonitrile synthesis by vapor-phase ammoxidation of 4-phenyl-o-xylene in the temperature range of 633 713 K have been studied. It has been shown that 4-phenylphthalonitrile is produced consecutively via 4-phenyl-o-tolunitrile as an intermediate. The 4-phenyl-o-xylene and 4-phenyl-o-tolunitrile conversion rates have been found to follow a half-order rate law in these components and to be independent of the oxygen and ammonia concentration. The kinetic behavior of the formation and consumption of byproducts are also discussed, their buildup rate equations are given, and possible reaction mechanisms are proposed. It has been revealed that the formation of the byproduct 4-phenylphthalimide at high ammonia concentrations is due to hydrolysis of 4-phenylphthalonitrile, carbon dioxide is produced via oxidation of 4-phenyl-o-xylene and decarboxylation of 4-phenylphthalimide, and 4-phenylbenzonitrile is produced from 4-phenyl-o-tolunitrile and 4-phenylphthalimide. Additional routes of the formation of 4-phenylphthalimide and CO2 from 4-phenyl-o-xylene appear at low ammonia concentrations.