Photon correlation spectroscopy has been used to study a monodisperse polystyrene latex during swelling by styrene monomer. Experimental results are presented which show that the latex became less stable and aggregated. A theory was developed to describe the results in terms of a static structure factor S(K) expressed as a sum of interference form factors between particles constituting the aggregates. By making the approximation that interference between nearest neighbours was the dominant term, it was possible to describe the state of aggregation of a system in terms of a mean coordination number. Where sufficient experimental data was available the theory was used to determine approximate values for both the swollen particle diameter and the degree of aggregation.