MSR IsoPro fuel is a specially formulated blend of isobutane and propane in an 80/20 ratio that enhances performance. It contains the purest isobutane with 5% or less n-butane than any other canister fuel, resulting in a clean-burning mixture. Compared to typical butane/propane blends, it maintains higher internal pressure at lower temperatures, allowing for superior cold weather performance and a more stable output throughout the life of the fuel canister.