A method using HPLC with tandem mass spectrometry (HPLC-MS/MS) is proposed and validated for quantitative determination of ubiquinone and ubiquinol in human blood plasma. Sample preparation used precipitation of proteins by a 1:1 mixture of i-PrOH–EtOAc. The investigated substances were analyzed on a Shimadzu 8040 triple quadrupole mass spectrometer in multiple reaction monitoring (MRM) mode for the main molecular ions of ubiquinone (m/z 880.7) and ubiquinol (m/z 882.7) using electrospray ionization (ESI) in positive- ion mode. Chromatographic separation used a Luna C18 column (150 × 4.6 mm, 5 μm) and gradient elution. Calibration curves showed a linear relationship in the concentration range 0.1 – 5 μg/mL. Validation of the proposed method confirmed its high sensitivity, specificity, accuracy, and precision. The method was successfully applied for determining ubiquinone and ubiquinol in blood plasma of conditionally healthy volunteers. [ABSTRACT FROM AUTHOR]