The aggregation of asphaltenes by pressure depletion both in a live crude oil and model systems of asphaltenes in toluene/pentane solvents is studied. Near-infrared spectroscopy utilizing a high-pressure NIR cell with a maximum operating pressure of 325 bar is used to study the onset of asphaltene aggregation. The NIR spectra are subject to a principal component analysis (PCA) in order to detect the asphaltene aggregation onset pressure. The effect of fluid compressibility on the NIR spectra is also demonstrated. The aggregation behavior of asphaltenes in model systems is shown to resemble the aggregation behavior for the crude oil. However, while the asphaltene aggregation in the crude oil is more or less completely reversible with repressurization, indications of only a partial redissolution are seen in the model systems. The kinetics of the redissolution. is quite slow. A time of 72 h to equilibrate at the original pressure of 300-bar was required to redissolve the asphaltene aggregates formed within the crude oil by depressurization of the sample. Near-infrared spectroscopy in combination with principal component analysis is shown to be an efficient tool in detecting both bubble points and asphaltene aggregation onset pressures in high-pressure systems.