Background Intraoperative regional myocardial acidosis (RMA) during cardiac surgery has been shown to be reflective of regional myocardial ischemia and an independent predictor of adverse postoperative outcomes. This study identifies the determinants of intraoperative RMA. Methods Intramyocardial tissue pH 37C in the anterior and posterior LV walls was measured in 641 adult patients during cardiac surgery. RMA at two intraoperative periods was quantified as integrated mean pH 37C < 6.35 during aortic clamping (AC) and pH 37C < 6.73 at the end of cardiopulmonary bypass (CPB). These pH thresholds were chosen because of their demonstrated relationship to long-term patient survival. Multivariate logistic regression models were constructed. An acidosis prediction score was constructed based on the factors determining RMA at the end of CPB. Results Independent determinants of RMA during AC were preoperative New York Heart Association class III/IV (P = .007), current smoker (P = .0088), pH 37C < 6.63 prior to AC (P < .0001), and intraoperative myocardial management technique (P = .0001). Independent determinants of RMA at end of CPB were ASA class IV/V (P = .0042), pH 37C < 6.63 prior to AC (P = .035), pH 37C < 6.35 during AC (P = .001), and total duration of CPB ≥ 212 minutes (P = .001). Conclusions RMA during cardiac surgery is determined by patient risk factors, the magnitude of preceding regional myocardial acidosis, and the duration of CPB.