It has been proposed that the addition of a tracer dye to a styrene emulsion polymerization system prior to the commencement of polymerization can cast light on the mechanism of latex particle nucleation. It was argued that the dye would be able to be leached out of the resulting latex by dialysis if a micellar mechanism were operative whereas leaching would be precluded if the particles were nucleated by a coagulative mechanism. Experiments to test this proposed method have been reported previously by Pashley and co-workers : they showed that leaching occurred with an oil soluble initiator, thus suggesting a micellar nucleation mechanism, but was not observed if the water soluble persulfate anions were used as the initiator, implying a coagulative nucleation mechanism. It is shown here that the interpretation of such experiments, performed in unbuffered systems, was vitiated by large changes in pH, such as occurs with persulfate anions as the initiator. It was found that, irrespective of the chemical nature of the initiator, dye could be leached from high pH latexes (pH > 8.0) but could not be leached from low pH latexes. Identical results were obtained whether or not the dye was added to the emulsion system prior to the commencement of polymerization or postadded after the latex had been formed. Moreover, the leaching of the dye from a high pH latex could be halted by lowering the pH. The results suggest that considerable caution must be exercised in deducing nucleation mechanisms using the dye leaching method.