The acid-catalysed decomposition of ethyl diazoacetate has been studied in the presence of sodium perchlorate, potassium chloride, potassium bromide, and potassium iodide. With halide ions some of the product is ethyl halogenoacetate rather than ethyl glycollate. The overall kinetics a n d product ratios have been measured and it has been shown that the halide ions take part in the rate-determining step which leads to halogenoacetate. The rate constants for this process for chloride, bromide, and iodide ions give a linear Swain-Scott nucleophilicity plot, but the rate constant for water is smaller than expected, which suggests that there is no change in mechanism and that it participates as a nucleophile in the glycollate reaction.