Trimethylsilylation of methylmalonic and ethylmalonic acids in the presence of headspace atmospheric oxygen is shown to produce the trimethylsilyl derivatives of pyruvic and 2-oxobutyric acids, along with 2-hydroxy-2-methylmalonic and 2-hydroxy-2-ethylmalonic acids, respectively. This may lead to overestimation of these keto acids, if they were not oximated in the original sample, and the mistaken reporting of the 2-hydroxymalonates.