Abstract Objectives We aimed to assess the intralumenal stability of oleuropein in human gastric and small intestinal contents. We additionally aimed to assess the stability characteristics of oleuropein in media simulating the intralumenal conditions. Methods The intralumenal stability of oleuropein was assessed in aspirates from the stomach and the upper small intestine of healthy volunteers collected under both fasted and fed state conditions and in media simulating the intralumenal environment. Key findings Oleuropein degraded in aspirates collected in the fasted state. When the initial concentration was about 50 μg/ml (close to expected intragastric concentration after single dose of commercially available products of oleuropein) the mean zero-order half-life of oleuropein in aspirates collected from the fasted small intestine was estimated to be 3.14 ± 0.08 h at 37°C (i.e. after oral administration in the fasted state, a substantial fraction of oleuropein degrades before reaching the intestinal mucosa). In contrast, oleuropein was stable in aspirates collected from the fed stomach; in small intestinal contents aspirated in the fed state the estimated zero-order degradation half-life was at least 12 h. Conclusions These data suggest that oleuropein should not have substantial intralumenal stability problems when administered in the fed state. Data collected in media simulating the intragastric and intraintestinal environment suggest that pH affects the stability of oleuropein only at low pH values (of about 2). At higher pHs degradation characteristics are at least partly affected by the presence of other scavengers of reactive oxygen species in the medium.