In this thesis, particle aggregation including homo- and heteroaggregaion in different ionic environments was investigated using electrophoresis and light scattering techniques combined with DLVO theory calculations. Chapter 1 presents a detailed introduction to particle aggregation, the experimental techniques involved, and the outline of the thesis. In Chapter 2, homoaggregation of amidine-modified polystyrene latex (AL) and sulfatemodified polystyrene latex (SL) particles in the presence of salts containing multivalent coions was studied. Chapter 3 presents homoaggregation of positive AL particles and negative SL particles in the presence of hydrophobic anions—tetraphenylborate (Ph4B–)and NaCl as background. In Chapter 4, heteroaggregation process between AL particles (300 nm in diameter) and SL particles (600 nm in diameter) was studied in two sets of solution systems containing multivalent ions. Chapter 5 demonstrates a strategy to measure the heteroaggregation rate over a wider range with the multi-angle light scattering technique.