Abstract Background To date, no scale has been validated to assess bubbles associated with bowel preparation. This study aimed to develop and assess the reliability of a novel scale – the Colon Endoscopic Bubble Scale (CEBuS). Methods This was a multicenter, prospective, observational study with two online evaluation phases of 45 randomly distributed still colonoscopy images (15 per scale grade). Observers assessed images twice, 2 weeks apart, using CEBuS (CEBuS-0 – no or minimal bubbles, covering < 5 % of the surface; CEBuS-1 – bubbles covering 5 %–50 %; CEBuS-2 – bubbles covering > 50 %) and reporting the clinical action (do nothing; wash with water; wash with simethicone). Results CEBuS provided high levels of agreement both in evaluation Phase 1 (4 experts) and Phase 2 (6 experts and 13 non-experts), with almost perfect intraobserver reliability: kappa 0.82 (95 % confidence interval 0.75–0.88) and 0.86 (0.85–0.88); interobserver agreement – intraclass correlation coefficient (ICC) 0.83 (0.73–0.89) and 0.90 (0.86–0.94). Previous endoscopic experience had no influence on agreement among experts vs. non-experts: kappa 0.86 (0.80–0.91) vs. 0.87 (0.84–0.89) and ICC 0.91 (0.87–0.94) vs. 0.90 (0.86–0.94), respectively. Interobserver agreement on clinical action was ICC 0.63 (0.43–0.78) in Phase 1 and 0.77 (0.68–0.84) in Phase 2. Absolute agreement on clinical action per scale grade was 85 % (82–88) for CEBuS-0, 21 % (16–26) for CEBuS-1, and 74 % (70–78) for CEBuS-2. Conclusion CEBuS proved to be a reliable instrument to standardize the evaluation of colonic bubbles during colonoscopy. Assessment in daily practice is warranted.