Nibroxane, 5-bromo-2-methyl-5-nitro-m-dioxane, is a topically effective antimicrobial agent with a broad spectrum of activity. Nibroxane is unusual in that it not only possesses high microbiocidal activity against Gram positive (Staphylococcus uureus) and Gram negative (Pseudomonas aeruginosa) bacteria but also against yeasts (Candida albicans) and moulds (Aspergillus niger). In addition, the 5-bromo-5-nitro-m-dioxanes have, as a class of compounds, the distinct advantage of being chemically stable over a wide pH range. This inherent stability, coupled with its broad spectrum of microbiocidal activity, makes nibroxane an excellent candidate as a preservative for cosmetic and pharmaceutical formulations.