The dynamics of shrinking of thin alkane droplets evaporating on isothermal surfaces has been studied in the case of complete wetting, where no pinning of the contact line is expected. As the resolution of the complete hydrodynamic equation is still an open problem, we propose a semiempirical approach, where only evaporation is taken into account. A careful analysis of the mathematical consequences of the form chosen for the evaporation term is given. The agreement with experiments is extremely good, suggesting that hydrodynamic contributions and Marangoni flows are probably negligible for the simple liquids considered.