Summary Background The importance of weakly acidic and weakly alkaline reflux in gastroesophageal reflux disease (GERD) is gaining recognition. Aim To quantify the proportions of reflux episodes that are acidic (pH < 4), weakly acidic (pH 4-7) and weakly alkaline (pH > 7) in adult patients with GERD, and to evaluate their correlation with symptoms. Methods Studies were identified by systematic PubMed and Embase searches. Data are presented as sample-size weighted means and 95% confidence intervals. Results In patients with GERD taking a proton pump inhibitor (PPI), 80% (76-84%) of reflux episodes were weakly acidic or weakly alkaline and 83% (78-88%) of reflux symptom episodes were associated with weakly acidic or weakly alkaline reflux episodes. In patients with GERD not taking a PPI, 63% (59-67%) of reflux episodes were acidic and 72% (57-87%) of reflux symptom episodes were associated with acid reflux episodes. Six studies presented data separately for weakly alkaline reflux, which accounted for < 5% of all reflux episodes, both on and off PPI therapy. Conclusions Weakly acidic reflux underlies the majority of reflux episodes in patients with GERD on PPI therapy and is the main cause of reflux symptoms occurring despite PPI therapy.