Colonic ischemia (CI) is a life-threatening complication after aortic surgery. Postoperative surveillance of colonic perfusion might be warranted. The aim of the present study was to evaluate the safety and feasibility of postoperative extraluminal pH measurement (pHe) using colonic tonometry after open abdominal aortic aneurysm (AAA) repair. Before closing the abdomen after open AAA repair, a tonometric catheter was placed transabdominally in contact with the sigmoid colon serosa, similar to a drainage catheter. Extraluminal partial pressure of carbon dioxide was measured postoperatively and combined with arterial blood gas analysis to calculate the pHe. The measurements were repeated every 4 hours with simultaneous intra-abdominal pressure measurements. The threshold for colonic malperfusion was set at pHe <7.2. A total of 27 patients were monitored, 12 had undergone surgery for ruptured AAAs and 15 for intact AAAs. Of the 27 patients, 4 developed clinically significant CI requiring surgery. All four cases were preceded by a prolonged (>5 hours) pHe <7.2 indicating malperfusion. A fifth patient, who, during monitoring, had had the lowest pHe of 7.21, developed mild CI with the onset after completion of monitoring, which was successfully managed conservatively. Seven patients who had had brief durations (<5 hours) of pHe <7.2 did not develop clinical signs of CI or any related adverse events. Measurements of pHe using colonic tonometry indicated malperfusion in all four patients who had developed clinically significant CI. A shorter duration of low pHe was well tolerated without any signs of CI. Measurement of pHe was safe and reliable for the surveillance of colonic perfusion after open aortic surgery, indicating a promising technique. However, larger studies are needed. Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.