The evolution of the detailed structure of agglomerate particles made by monomer-cluster coagulation is described by single-particle trajectory calculations based on the ＇＇Langevin differential equation＇＇. This method is used also to determine the collision frequency between agglomerate and spherical particles undergoing Brownian coagulation from the free molecular to the continuum regime. A new analytical function is proposed for the collision frequency between agglomerate and spherical particles.