Abstract Gastro-oesophageal reflux (GOR) is common in preterm infants. Combined multichannel intraluminal impedance and pH monitoring (pH-MII) is emerging as an useful tool to study both acid and non-acid GOR in this population. We aimed to highlight main advantages and limits of pH-MII in preterm infants and to test whether the inclusion of GOR episodes detected only by pH monitoring details better the features of GOR. Fifty-two symptomatic preterm infants underwent a 24-hour, continuous and simultaneous measurement of pH-MII. Each layout was analyzed using two different options: option 1 included GOR episodes detected by MII and then classified as acid or non-acid according to the associated pH change; option 2 included GOR episodes detected by MII and also GOR episodes detected only by pH sensor. By adopting option 1, a total number of 2834 GOR episodes was detected by MII: 2162 of them were characterized as non-acid and 672 were characterized as acid. The median (range) number of acid MII-GOR episodes was 10 (1–52); the median (range) number of non-acid MII-GOR episodes was 36.5 (2–119). Median (range) acid MII-GOR-bolus exposure index was 0.28% (0.02–2.73%); median (range) non-acid MII-GOR-bolus exposure index was 1.03% (0.06–38.15%). By adopting option 2, an average of 53.2 acid GOR episodes and an average of 11% oesophageal exposure to acid GOR more than by option 1 was detected. An accurate and detailed description of GOR in preterm infants can be obtained only by including in the analysis all acid GOR episodes detected by pH sensor.