In-situ monitoring of particle characteristics is of general interest for both natural and engineered aquatic systems and of particular interest in studying the fragile floc that typically is formed by the addition of chemical coagulants. A nonintrusive photographic technique coupled with digital image processing for in-situ analysis of aggregates formed by the addition of alum [Al2(SO4)3·18H2O] to lake water and a montmorillonite clay suspension is described. The technique is unique in that there is no need for sample collection and handling. The analysis method is used to test the hypothesis that charge-neutralization and sweep-floc mechanisms produce fundamentally different particle characteristics, including differences in fractal dimension. For comparative purposes, particle characteristics prior to coagulant addition also are reported. It is found that fractal dimension is lower for sweep-floc coagulation where larger and more irregular aggregates are produced. The results presented here provide ins...