Abstract: The peroxide content of a non-aqueous medium is measured by a system including separate supplies for two carrier solutions, a first one containing an acid for acidifying the sample and the second containing an iodide compound for reacting with the peroxide(s) in the sample and producing a free iodine ion, such as I.sub.3.sup.-, and a flow-through detector, such as a colorimetric device. The sample and first carrier solution streams are controlled by a sample injection valve which is movable between a load position wherein the sample stream is routed to a sample waste collector via a sample loop and the first carrier solution stream passes through a first carrier solution conduit and is combined with a stream the second carrier solution prior to passing through the detector and a measure position wherein a slug of the sample is introduced into the first carrier solution conduit, mixed with the first carrier solution and combined with the second carrier solution prior to passing through the detector.