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The emulsion polymerization of 2-methyl-5-vinylpyridine (MVP) initiated by potassium persulphate in the presence of two anionic emulsifiers, sodium lauryl sulphate (NaLS) and sodium oleate, was investigated. The effects of persulphate and emulsifier concentration on the rate of polymerization are not those predictable on the basis of the theories of Harkins and Smith-Ewart. MVP polymerizes with a high rate in water solution and in emulsion even in the absence of emulsifier. A few emulsion polymerizations carried out with various NaLS concentrations near the critical micelle concentration showed that the presence of emulsifier in micellar form does not substantially increase the rate of polymerization, as predicted by the above theories. The behaviour of MVP is probably related to its moderate water solubility, which is 1·04 per cent by weight at 60°. Some experiments with seeded latexes confirmed that the water phase plays a predominant is not exclusive role as locus of polymerization.