Fuels for military applications have more severe restrictions than for the usual commercial consumer. One of the most stringent and difficult requirements to meet is that of fuel storage stability. Storage instability is defined in terms of solids formation which can plug nozzles and filters. Materials added as fuel blending stocks must be chemically stable and not induce instability in the fuel to which it is added. In the present research, a soybean-derived fuel was added in concentrations up to 20% in both stable and unstable petroleum middle distillate fuels. The storage stability of the mixture was tested by ASTM 5304. The soy-fuel mixtures proved stable in the stable fuel and reduced the instability in the unstable fuel significantly.