Process technology for selectively isomerizing vinylidene olefin to tri-substituted olefin is described. The vinylidene olefin, normally in admixture with other types of olefins, especially linear 1-olefins, is treated with anhydrous hydrogen bromide under anhydrous conditions and in the absence of molecular oxygen and free radical initiator. The contacting period, which can be a matter of minutes, is sufficient to selectively isomerize the vinylidene olefin to tri-substituted olefin. If the process is conducted properly, little if any hydrobromination occurs.