Ozonation of aqueous solutions of dimethylamine (DMA) leads to the formation of nitrosodimethylamine (NDMA). The yield of reaction is low (below 0.4% in relation to DMA) and increases with increasing pH. Contact time, ozone/DMA ratio and radical scavengers are other variables controlling the yield of reaction. Data from the literature and observed ozonation by-products suggest that nitrosation of DMA might be responsible for nitrosamine generation. NDMA can be recognized as a by-product of ozonation of DMA in water, which is formed in a specific, but reasonable, range of ozone/DMA ratios. The reaction may have potential importance for water treatment technology assuming reasonable micrograms per liter of DMA concentrations in raw waters.