14C02 output from carboxy-14C-labelled aliphatic carboxylic acids is measured in the micro-scale Kolbe reaction. Irrespective of whether rats were dosed with 1,1-dichloro[1-14C]ethylene or with chloro[1-14C]acetic acid, 1 mol.equiv. of the resulting thio[14C]diglycollic acid yields by electrolysis approx. 0.7 equiv. of 14CO2, which is interpreted in terms of the labelling of one of the carboxylic acid groups of thiodiglycollic acid. This observation provides important evidence concerning thiodiglycollic acid biosynthesis from 1.1-dichloroethylene.