These microspheres show a size distribution which is appropriate for methods that need particle size uniformity between monodisperse (highly uniform, <5% CV**) and polydisperse (widely distributed, <12% CV). These include instruments for light blockage, light diffraction, acoustical imaging, electrolyte displacement or other methods used for analysing wide size range materials. The material is composed of polystyrene cross-linked with divinylbenzene. This gives the particle good durability and chemical stability. They can be washed in water-alcohol mixtures and resuspended in water, air or similar media. The density of polystyrene is 1.05 g/cm3, and the index of refraction is 1.59 at 589nm (25C). Microspheres in this series are packaged as an aqueous suspension in 15 mL dropper-tipped vials.