Impedance monitoring determines reflux composition as liquid and/or gas, and we assess with impedance the effect of posture and a meal on reflux composition. Twenty-nine gastro-oesophageal reflux disease (GORD) patients and 10 normal volunteers underwent simultaneous impedance, manometry and pH for three 40-min periods, each 20-min upright, 10-min left side and 10-min right side. One period was fasting, and two postprandial. We found that reflux event composition on the right side was liquid-only near exclusively (93%). In contrast, 85% of reflux on the left side and 74% upright was gas-only or liquid and gas (P < 0.001). More gas-only reflux occurred fasting (80%) than postprandial (P < 0.01). Reflux composition was similar in the study groups and two postprandial periods (P > 0.25). After confirming gas reflux suppression on the right side, a pilot study assessed gastric anatomy as a cause. Five achalasia patients had fluid injected into the stomach at endoscopy, and the EG junction was submerged below liquid-only on the right side. We conclude that reflux is nearly always liquid-only on the right side where the EG junction may be below fluid. Reflux fasting is usually gas-only. Different posture and fasting times may (i) explain variations in acid exposure during pH monitoring and (ii) promote or prevent GORD symptoms.