Method and device for predicting the flocculation threshold of asphaltenes contained in hydrocarbon mixtures. The method comprises the following stages: determining the refraction index (nA) of several asphaltenes used as reference asphaltenes; determining experimentally, for predetermined thermodynamic conditions, the variation of solubility index (&dgr;) in connection with the refraction index of hydrocarbon constituents including light hydrocarbons and reference asphaltenes; deducing therefrom a correlation relation modelling this variation of solubility index (&dgr;). These preliminary operations being performed, refraction index (nA) of the asphaltenes of the hydrocarbon mixture is determined under the same thermodynamic conditions and by reference to said correlation relation, solubility index (&dgr;) of the asphaltenes of said mixture is deduced therefrom; and from the solubility index of the asphaltenes of said mixture and from a thermodynamic equilibrium model, the flocculation threshold thereof is directly predicted. Applications: fast laboratory determination of asphaltene flocculation risks likely to hinder production and transport of crudes so as to quickly implement solutions intended to prevent them.