To determine the vaporization order of (the components in) crude oils, the density, the viscosity and the chemical composition of a light and a heavy crude oil were studied as a function of the burning efficiency. An experimental series of small scale in-situ crude oil burns on water were conducted with the two crude oils. Chemical analyses of the burned residues showed that the components in crude oils vaporize in order of decreasing volatility and the depletion rate of components generally decreased with increasing molecular mass. Ultimately, this means that the burning efficiency of a crude oil burning on water can be related to fire dynamics principles, irrespective of its chemical and physical properties. The relative abundance of pyrogenic PAHs in the burned residues increased up to a maximum of 2600% for the light crude oil and 9100% for the heavy crude oil. Increased abundances of the pyrogenic PAHs were caused by the formation of the pyrogenic PAHs during the burning and not by an increase in concentration in the burned residues. Overall, the results provide relevant data for predicting the effectiveness of in-situ burning of crude oil as oil spill response method, both in terms of its burning efficiency and its environmental impact.