A model is proposed for estimating the rate of vaporisation of LNG and LPG cryogen mixtures spreading on unconfined water surfaces. The model is used to examine the influence of chemical composition on the vaporisation rate of LNG and LPG during spreading. Calculations have been performed whereby the vaporisation rate of the LNG and LPG mixtures has been compared to the vaporisation of pure methane and propane, respectively, under the same initial conditions. The detailed results indicate that the vaporisation rate of LNG mixture is markedly different to that of pure methane, while the vaporisation rate of LPG mixture is similar to that of pure propane. The difference can be attributed primarily to the contributions of the direct and indirect component of the total, differential, isobaric latent heat to the boiling process. For LNG, as the liquid mixture gets rich in ethane, the total, differential, isobaric latent heat increases rapidly, leading to a large decrease in the vaporisation of LNG compared to pure methane. For LPG, because of the shape of the phase envelope, only a small increase of the total latent heat and the boiling temperature is observed and consequently the change in the vaporisation rate is marginal. The overall results suggest that treating an LNG spill as a pure methane spill results in underestimation of the total spillage time of the order of 10-15% and in qualitatively wrong dynamics of the rate of vapour formation; thus, warranting a full treatment of the thermodynamics of the mixture.