Objective To compare the diagnostic value of using 24-hour combined esophageal multichannel intraluminal impedance and pH monitoring (MII-pH) in the diagnosis of gastro-esophageal reflux (GER) and pH monitoring alone in critically ill patients. Methods A prospective observational study was performed including 116 critically ill adult patients admitted to ICU of Peking Haidian Hospital from Jul. 2013 to Dec. 2014. All the patients underwent 24-hour combined MⅡ-pH monitoring. GER episodes were recorded and its pH was recorded (acidic, weakly acidic and weakly alkaline) and its composition was recorded (liquid, mixed and gas reflux). The results of the MⅡ-pH and the pH were monitored and compared. The demographic characteristics and clinical information were recorded. Results MⅡ-pH was monitored for 5024 episodes of GER in 115 of 116(99.1%) patients, with a mean of 43.28±3.96 episodes per patient (median, 34 episodes; range, 0-196 episodes). The pH monitoring detected 1868 episodes (100% acid) in only 54 of 116(46.6%) patients, with a mean of 7.66±1.65 episodes per patient (median, 0 episodes; range, 0-81 episodes). The number of episode of all reflux and liquid reflux diagnosed by pH monitoring alone was less than those diagnosed by MⅡ-pH monitoring (P=0.000), and there was no correlation in the episodes number of all reflux and liquid reflux between the two techniques (r=0.119, 0.231). Only a moderate correlation was found in the number of episodes of acidic reflux between the two techniques (r=0.656). Conclusion MⅡ-pH monitoring is more sensitive than pH monitoring alone for establishing the diagnosis of GER. DOI: 10.11855/j.issn.0577-7402.2016.05.12