The detailed understanding of droplet coalescence is important for the accurate description of liquid/liquid dispersions. A test cell is designed which enables serial examinations of the random coalescence process with high repetition rate, good observability and accuracy of experimental parameters. Within this rectangular test cell a rising droplet collides with a pendant one, while recorded by a high speed camera. The gained experimental data allows a validation and further development of appropriate models. The investigated parameters in this work are the drop size and the superimposed mass transfer influencing the coalescence probability. These examinations were carried out in the EFCE standard test system toluene/acetone/water. The effect of varying the drop size seems to be interfered by the different rising velocities due to buoyancy. Introducing a transferring component has a significant impact on the coalescence process. A transfer direction from disperse to continuous phase results in a coalescence probability of almost 100%, whereas the reverse mass transfer direction induces a repulsion of nearly all droplets. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.