Regurgitation and reingestion (R/R) of foodstuffs is a common abnormal behaviour in captive western lowland gorillas (Gorilla gorilla gorilla) and certain other non-human primates, but it is not part of their normal feeding mechanism. It is similar to a behavioural problem seen in humans, human rumination syndrome (HRS), which can occur in association with anxiety or a poor relationship with a caregiver. Patients with HRS often regurgitate stomach acid with the food matter which can result in clinical problems; until this study, it was not known if stomach acid is regurgitated by gorillas also. Thus, samples of regurgitated matter were collected opportunistically and non-invasively, and pH was measured using an electronic meter. Results were compared with the pH of samples of the original food eaten by the gorillas, and show that regurgitated food has significantly higher acidity than the originally-ingested meal. By comparison, samples of saliva were collected from gorillas opportunistically, in the absence of recent ingestion, and were found to be alkaline and, thus, saliva should not have contributed to the increased acidity of regurgitated matter. The results imply that stomach acid is being regurgitated, as in human patients with potentially-injurious rumination syndrome, and it is indicative of sub-optimal welfare. Causes and effects of R/R should be investigated further, to lead to potential treatment and prevention and to promote the welfare of captive gorillas.