We present a new technique-nuclear magnetic resonance dispersion of distributions (NMRDD)-and we demonstrate that it is a particularly sensitive probe of aggregation in asphaltene-rich crude oils. The technique is based on the measurement of the NMR relaxation distributions at different magnetic fields, i.e., at different proton Larmor frequencies. Because the NMR relaxation rates are controlled by molecular fluctuations at the Larmor frequency, these measurements probe the presence of aggregates that tumble at characteristic rates near that frequency. Our experimental results span the Larmor frequencies from 10 kHz to 40 MHz, which is the range relevant for rotation times of asphaltene and resin aggregates in crude oil. The presence of large aggregates is evident by visual inspection of the data, and aggregate sizes for small and medium size aggregates of the order of several nanometers can be extracted without recourse to any specific models of asphaltene interactions. The technique is noninvasive and nondestructive and can be used for monitoring and study of the onset of aggregation at the preflocculation stage in unstable crude oils.