An experimental study to define a practical methodology that will serve to assess the burning of crude oils on a water sub-layer by means of a bench scale procedure is presented. A modified ASTM-E1321 (LIFT) is combined with flash point measurements to extract fuel properties by means of an existing theoretical formulation. The fuel and water layers are treated as a thermally thick material with combined properties to be able to obtain an analytical solution for an ignition delay time and a flame spread velocity. The experimental results are then correlated to the theoretical formulation to obtain the "fire properties" described in the ASTM-E1321 standard. Five different parameters have been identified that describe the capability of a liquid fuel to sustain a flame: the critical heat flux for ignition, q(0)",(ig), an ignition temperature, T-ig, the heat contribution of the flame, phi, a thermal efficiency, a/h(T) and the thermal inertia, rootk rhoc/a. The methodology is used to evaluate the effect of weathering on the flammability of Alaska North Slope (ANS) and Cook Inlet crude oils. The critical heat flux for ignition was found to be a strong function of the weathering level and a weak function of the fuel layer thickness. The ignition temperature depends on both the weathering level and the fuel layer thickness. Thermal efficiency, heat contribution from the flame and thermal inertia remained invariant with the weathering level and fuel layer thickness.