AbstractThe influence of residual styrene monomer and benzaldehyde on the surface properties of polystyrene latices has been considered. These materials are commonly found after emulsion polymerisation and their complete removal by dialysis is almost impossible. Steam stripping at reduced pressure (nitrogen atmosphere,<350K) can be used to remove the last traces of styrene and benzaldehyde from polystyrene, but this process results in some degree of hydrolysis of the surface sulphate groups.Latices are easily contaminated by microorganisms, especially those which are airborne. This can occur during cleaning and/or storage. Microorganisms can survive and multiply in suitable latex environments and can result in the destabilisation and subsequent flocculation of the latex. Controlled experiments with a latex dosed with various levels of a penicillium type fungus demonstrated the possibility that enzymes (classified as aryl sulphate sulphohydrolases ‐ E. C. 3.1.6.1.) were produced which resulted in the rapid hydrolysis of surface sulphate groups. Fungal growth was accompanied by the release of weak acids and metabolites which were titrated in the aqueous phase.Careful consideration should be given to the conditions of latex preparation, cleaning and storage, e.g. use of autoclaved water and γ‐irradiation techniques, which will reduce the possibility of microbial contamination. Routine tests for bacteria and fungi should be carried out by plating on microbiological media, since the usual tests for protein such as the Folin‐Ciocalteu are insensitive at the low levels of contamination which may be p