Microspheres were produced by rotary spray congealing molten suspensions composed of poloxamer 188 and a single solid excipient. Six different excipients, spanning a range of particle size, density and morphology were used. The size distribution, true density and shape of the resulting congealed particles were assessed. Disk rotation speed had the most significant effect on microsphere size, with higher speeds yielding smaller congealed particles. At low disk speeds, congealed particle size decreases with increasing dispersed solid density. Over the range investigated, neither suspended solid size nor morphology had a significant effect on the congealed microsphere size. Measurements of the density of the microspheres suggest that the solids were homogeneously spread across the different congealed particle sizes. This suggests that rotary atomization of poloxamer 188 suspensions is an effective method for producing spherical particles for encapsulation for a wide variety of drug particle sizes, shapes and densities-with disk speed being used to control the mean size.