Abstract Heavy or extra heavy crude oils cannot be transported by pipeline without a prior reduction of their viscosity. This is commonly obtained by blending the oil with light hydrocarbons. In that case, the resulting viscosity of the mixture depends only on the dilution rate, and on the respective viscosities and densities of the oil and of the diluent. Classical diluent are light crudes, condensates, naphtha. Availability of the diluent and its possible recycling needs of course to be taken in account. In this work we have studied the influence of composition of the diluent on the viscosity reduction efficiency. In particular we show that the addition of a polar solvent to a solution of asphaltenes in toluene acts on the colloidal structure of the asphaltenes. The relative viscosity of the solution decreases, as well as the size of the aggregates of asphaltenes. In the same way, by mixing hydrocarbons and solvents owning polar functional groups in their molecule, it is shown that the efficiency of the dilution of heavy crude oils is enhanced. Hansen's theory can be used to screen the solvent efficiency. At constant dilution rate, the higher the polarity parameter or the hydrogen bonding parameter of the solvent, the greater the relative viscosity reduction of the diluted crude oil. Nevertheless, solvent owning high hydrogen bonding are generally more viscous than hydrocarbons. The influence of their interactions with the asphaltenes is hidden when the results are expressed in absolute viscosity. Polar solvents giving few hydrogen bonding give a significant reduction of the viscosity of the diluted crude oil. From an economical point of view, any improvement of the efficiency of the thinner could be a benefit for the process, by reducing the amount of solvent needed to get an acceptable viscosity, and therefore allowing a greater quantity of crude to be transported.