Rationale: Weaning from venovenous extracorporeal membrane oxygenation (VV-ECMO) is based on oxygenation and not on carbon dioxide elimination. Objectives: To predict readiness to wean from VV-ECMO. Methods: In this multicenter study of mechanically ventilated adults with severe acute respiratory distress syndrome receiving VV-ECMO, we investigated a variable based on CO(2) elimination. The study included a prospective interventional study of a physiological cohort (n = 26) and a retrospective clinical cohort (n = 638). Measurements and Main Results: Weaning failure in the clinical and physiological cohorts were 37% and 42%, respectively. The main cause of failure in the physiological cohort was high inspiratory effort or respiratory rate. All patients exhaled similar amounts of CO(2), but in patients who failed the weaning trial, [Formula: see text]e was higher to maintain the Pa(CO(2)) unchanged. The effort to eliminate one unit-volume of CO(2), was double in patients who failed (68.9 [42.4-123] vs. 39 [20.1-57] cm H(2)O/[L/min]; P = 0.007), owing to the higher physiological Vd (68 [58.73] % vs. 54 [41.64] %; P = 0.012). End-tidal partial carbon dioxide pressure (Pet(CO(2)))/Pa(CO(2)) ratio was a clinical variable strongly associated with weaning outcome at baseline, with area under the receiver operating characteristic curve of 0.87 (95% confidence interval [CI], 0.71-1). Similarly, the Pet(CO(2))/Pa(CO(2)) ratio was associated with weaning outcome in the clinical cohort both before the weaning trial (odds ratio, 4.14; 95% CI, 1.32-12.2; P = 0.015) and at a sweep gas flow of zero (odds ratio, 13.1; 95% CI, 4-44.4; P < 0.001). Conclusions: The primary reason for weaning failure from VV-ECMO is high effort to eliminate CO(2). A higher Pet(CO(2))/Pa(CO(2)) ratio was associated with greater likelihood of weaning from VV-ECMO.