Abstract A dichloroacetate-selective electrode with a liquid membrane of tetraoctylammonium dichloroacetate dissolved in o-nitrotoluene is described. The electrode exhibits near-Nernstian response to dichloroacetate activity from 0.01 to 2 × 10 −5 M, in the pH range 3.5–10. A potentiometric determination of chloramphenicol in pharmaceutical preparations is reported; the dichloroacetate released from the antibiotic after alkaline hydrolysis at 30°C is measured directly. Chloramphenicol in the range 5–15 mg can be determined with an average error of about 2.1%. The results obtained for real samples compare favorably with those obtained by a polarographic procedure.