Injection-molded Krayton synthetic rubber. These grips are made from the same material used in dirt bike and BMX grips: Kraton Rubber. They are softer and more comfortable but also wear quicker. Just like a dirt bike, plan on changing them often, depending on how hard you ride and in what conditions.
We recommend using WD-40 to install the grips and suggest letting them dry overnight. Adding a ring of safety wire on each end is a standard moto practice.