Knowledge of the chemical functions existing in bitumens represents an important step in the characterization of their aging processes. A new investigation method based on a combination of precipitation and IEC separations followed by both qualitative and quantitative Si-29-NMR spectrocopy is presented. This method is proved to be an efficient procedure for the determination of the acidic fractions of these high-boiling petroleum fractions. It has been demonstrated that phenols, carboxylic acids, and other oxygenated functions were present. A correlation between the acidic character and the aging of bitumens is presented as a prediction of the rate of evolution of these materials.