Perikinetic coagulation experiments are reported which were performed under 1g and µg conditions during a flight in a sounding rocket, for dispersions of polystyrene, quartz, and amorphous silica particles. Coagulation rates for dispersions of polystyrene, quartz, and amorphous silica are found to increase significantly under µg conditions compared to 1g conditions. Another set of experiments was performed at 1g, 2g, 4g, and 7g, with different densities of the continuous phase; also a mixture of two different polystyrene latices was used. The most pronounced effect of gravity on coagulation rate is found for dispersions with a small density difference, on going from 1g to 2g. In this regime a significant decrease in coagulation rates is observed. For the latex mixture at pronounced density differences, gravity-induced coagulation was observed; however, the aggregates formed did not have a lasting contact. The rate constant calculated under µg conditions approaches the theoretical value of von Smoluchowski. By means of a video analysis of the perikinetic coagulation process, the formation of doublets was studied. The "interaction time" for two particles was found to be longer for the density-matched dispersion. Doublets of particles are easily disrupted, and at 1g, free convection currents were observed even at small temperature differences in the system. Copyright 1998 Academic Press.