Mountain Gear says:
MSR IsoPro Canister Fuel 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.
Historic Range: $3.95-$113.85
Sold in 4, 8 and 16 oz. canisters to meet the demands of any stove or trip. The 4 oz. canister is sized to fit inside the 1.0L WindBurner and Reactor Stove Systems.