The structure of the disperse phase of crude oils and liquid petroleum products is determined by the characteristics of formation of molecular aggregates of asphaltenes. The characteristics of these processes in dilute solutions of crude oil in toluene were investigated by measuring the proton spin-spin relaxation time as a function of the concentration of asphaltenes. These measurements allowed explaining the conditions of formation of dimers and more complex aggregates of asphaltene molecules. The data obtained by the NMR-relaxation method are confirmed by the results of measurements of the viscosity and optical absorption of the petroleum solutions investigated.