This chapter describes the use of reversed-phase HPLC with multichannel coulometric electrochemical detection for the routine, sensitive, and simultaneous measurement of oxidized and reduced CoQ10 and CoQ9 in human plasma and serum. Analytes are first resolved chromatographically prior to electrochemical detection using three serially placed flow-through coulometric sensors set for oxidation-reduction-re-oxidation. Such electrochemical manipulation of analytes not only improves selectivity and specificity (decreasing the likelihood of co-elution), but also leads to improved sensitivity and decreased noise. The method is completed in ,18 min, shows excellent linearity, good intra-day (% RSD = 1.2-2.3) and inter-day (% RSD 2.2-3.9) precision, and has a limit of detection to low pg levels (on column). This approach was used to measure oxidized and reduced CoQ10 and CoQ9 in 30 human plasma samples, and oxidized and reduced CoQ10 in 10 human serum samples (NIST Micronutrients Measurement Quality Assurance Program for fat-soluble vitamins).