The VAPEX (vapour extraction) process is a new technique for the recovery of highly viscous heavy oil and bitumen. This process involves injection of vapourized hydrocarbon solvent into heavy oil and bitumen reservoirs and production of the resulting solvent-diluted oil that drains by gravity in a horizontal well. Research has shown that this process is highly efficient and that different solvents give different results. In this paper, six different solvents were added to several oils of different viscosities and densities. The solvents were added in different ratios to each of the oils and NMR spectra were obtained. The mixture of solvent and heavy oil or bitumen produces a spectrum that is distinctly different than that of the solvent or oil alone. From the shape and amplitude of the NMR spectra, one can calculate the amount of solvent present. Furthermore, one can predict the viscosity of the mixture without any additional viscosity measurements. As asphaltenes precipitate with the addition of solvent(s), one can correlate the amount of asphaltene reduction to changes in the NMR spectra. In this manner, NMR can possibly be used to show the asphaltene precipitation of different oils in the presence of solvent(s). By measuring the amount of asphaltene precipitation, NMR can also provide an indication of in situ upgrading of the oil that occurs with the addition of solvent(s). Using NMR as an analysis tool, the effect of the different solvents on viscosity reduction and asphaltene precipitation is quantified.