The phase equilibria of supercritical CO2-alkane-alcohol mixtures were studied using online gas chromatography. Gas chromatograph design criteria and quantitative detector calibration methods are presented. An adapted manual calibration injection technique eliminated liner overload and pressure wave effects. Observation of the cell contents has proven essential for accurate vapor phase sampling. Visuals, often not visible to the naked eye, show that sampling can disrupt an equilibrated high-pressure system even though pressure and temperature remain constant within 0.1 bar and 0.01 degrees C. Such disruptions may manifest in one of three phenomena: global mist formation, localized mist formation, or no-warning droplet formation.