MSR IsoPro is a performance-boosting, 80/20 blend of isobutane and propane, with the purest isobutane (5% or less n-butane) of any canister fuel. This clean-burning formulation maintains higher internal pressure at lower temperatures than standard butane/propane mixtures, delivering superior cold weather performance and more consistent output over the life of the canister. Size: The 4 oz. canister is sized to fit inside the 1.0L WindBurner® and Reactor® Stove Systems. Improved Performance: 80/20 blend of isobutane and propane delivers superior performance throughout the life of the canister and in colder temperatures. Clean-Burning: Zero-maintenance combustion. Fuel Gauge: Built-in float gauge determines how much fuel is left (4 oz. and 8 oz. canisters). Recyclable: Consult your local recycling center for regulations.