The direct application of nanoparticles as nonsupported adsorbents and catalysts is of high interest since they offer high surface areas with reduced mass transfer limitations. However, the natural tendency of these materials to aggregate, even faster when at high temperatures, makes the agglomeration process an important phenomenon to be studied, understood and, eventually controlled. A method to obtain the kinetics of nanoparticle agglomeration processes is presented in this paper. This analysis was based on the change of particle diameter during aggregation. The kinetic expression was validated with a series of experiments where the growth of Fe2O3 nanoparticles immersed in base oil was followed at different times, temperatures, and particle concentrations. Results revealed the nature of the particle agglomeration process in the ranges of the experimental conditions; they indicated that physical adhesion, more than chemical binding, is the determining mechanism for agglomeration of Fe2O3 nanoparticles immersed in base oil.