Heavy and extra-heavy crude production is increasing in Mexico, and this fact entails to deal with several issues especially due to transport. Some physical processes have been employed to reduce crude viscosity and friction drag in pipelines to achieve flow enhancement, and chemical products are also applied for this purpose. Even though several physicochemical processes are involved, the real intermolecular effects are barely known. In this work, Nuclear Magnetic Resonance Spectroscopy (NMR) was used to evaluate compositional changes of extra-heavy crude oil in which certain amount of a viscosity reducer was added. Adding 5% of the product, 66% viscosity reduction was obtained at 25 °C and the NMR spectra of this sample showed overlapping signals at 0.5 and 3.5 ppm, typical of crude oil, as well as a singlet at 3.6 ppm and a multiple signal at 5.34 ppm, which was different for the sample not dosed; this behavior is associated to the presence of the viscosity reducer in the mixture. Also, it was corroborated that the dosed product aggregates to the medium fraction of the crude without phase separation.