In suspension polymerisation it is sometimes necessary to add material to the reactor after the reaction has started. When that happens, the new material and existing drops can remain segregated for significant amounts of time. Hashim and Brooks (Chem. Eng. Sci. 57 (2002) 3703) showed that the viscosity of drops affects both their sizes and their rates of coalescence. In the work reported here, further clarification of the drop mixing process is achieved by using pairs of stabilised dispersions. Solutions of polystyrene (PS) in styrene formed the dispersed phase and the tracer die technique was used to determine the extent of drop mixing. Drop mixing rate increased when the polymer content of all the drops increased from 0 to 5 wt% but further increases in polymer content lead to a reduction in mixing rate. Drop viscosity affected the mixing rate both directly and indirectly, because viscosity affected drop size and that influenced the drop-mixing rate. With increased polymer content, the larger drops made little contribution to the mixing. Experimental results were compared with the prediction of a model developed previously by Alvarez et al. (Chem. Eng. Sci. 49(1) (1994) 99). The model is consistent with the initial coalescence rates that were deduced from the experimental measurements. Drops containing 0 wt% PS mixed more quickly with drops containing 5 wt% PS than with drops containing 10 wt% PS. In those cases, the initial drop size distribution was relatively wide but, gradually, became narrower (the larger drops disappeared). With no polymer in the dispersed phase, the mixing of pre-dispersed drops was slower than the mixing that occurred when a batch of non-dispersed material was added to a stabilised dispersion (i.e. batch mixing). But, with 10 wt% of polymer in the dispersed phase, the mixing rate of two stabilised dispersions was similar to the batch mixing rate (even though the added polymer solution was not initially protected by the stabiliser). Mixing of two stabilised dispersions, with drops containing different amounts of PS, indicated that drop viscosity influences the mixing rate more than the difference in drop sizes. (C) 2004 Elsevier Ltd. All rights reserved.