The invention relates to a robust estimation of biodiesel blend ratio for alternative fuel combustion. A method to control an internal combustion engine includes operating the engine with a fuel blend of a first fuel and a second fuel, monitoring a value of a first combustion parameter during engine operation, monitoring a first value for a second combustion parameter during engine operation, determining a second value for the second combustion parameter in accordance with a predetermined correspondence among the first combustion parameter, the second combustion parameter, and a predetermined fuel blend of the first fuel and the second fuel, determining the fuel blend based upon a difference between the first and second values for the second combustion parameter, and controlling the engine based upon the fuel blend.