Purpose: To study the prevalence, characteristic, outcome, and acid-base biomarker predictors of outcome for different acidemia subtypes. Methods: We used national intensive care databases from three countries and classified acidemia subtypes as metabolic (standard base excess [SBE] < −2 mEq/L only), respiratory (PaCO2 > 42 mmHg only), and combined (both SBE < −2 mEq/L and PaCO2 > 42 mmHg) based on blood gas analysis in the first 24 h after ICU admission. To investigate acid-base predictors for hospital mortality, we applied the area under the receiver operating characteristic curve approach. Results: We screened 643,689 ICU patients (2014–2018) and detected acidemia in 57.8%. The most common subtype was metabolic (42.9%), followed by combined (30.3%) and respiratory (25.9%). Combined acidemia had a mortality of 12.7%, compared with 11% for metabolic and 5.5% for respiratory. For combined acidemia, the best predictor of hospital mortality was pH. However, for metabolic or respiratory acidemia, it was SBE or PaCO2, respectively. Conclusions: In ICU patients with acidemia, mortality differs according to subtype and is highest in the combined subtype. Best acid-base predictors of mortality also differ according to subtype with best performance for pH in combined, SBE in metabolic, and PaCO2 in respiratory acidemia.