The oxidation of 3-carboxy-3-hydroxy pentanedioic acid (citric acid) by N-bromosuccinimide (NBS) in an acetic acid/sodium acetate buffered medium (pH 3.6-5.2) was carried out at 308 K with a constant ionic strength. The rate was first order with respect to both 3-carboxy-3-hydroxy pentanedioic acid and NBS. The rate was inverse first order in succinimide, the reduction product of N-bromosuccinimide. The rate decreased with an increase in pH. A mechanism consistent with the experimental results was proposed and supported by kinetic orders, spectrophotometric evidence, negative entropy of activation, and observed zero ionic strength of the medium. The reaction was carried out at four different temperatures, and the activation parameters were determined.