A Monte Carol simulation technique developed describes dispersed-phase systems with emphasis on coagulation and aggregation. The method does not use particle trajectories, but is based on the transformation of known collision frequencies into collision probabilities of particle pairs. The particle evolution was computed as a stochastic game, computing the time step after each collision. The simulations were validated by comparing with exact mathematical solutions for aggregation of solid particles and with numerical solutions based on sectional methods for coagulation of droplets. The direct simulation Monte Carlo (DSMC) method is advantageous, because the simulation of complex, multidimensional systems results in very elaborate models when using sectional models and is implemented very easily. Two examples of industrial importance are chemical reaction in coagulating droplets and coating of particles with small solid particles.