Midazolam is a commonly used sedative for patients on extracorporeal membrane oxygenation (ECMO). However, its long context-sensitive half-life is of concern. Remimazolam may be a suitable alternative. This study compares the sequestration of remimazolam and midazolam in an in-vitro ECMO circuit. Sixteen in-vitro ECMO circuits were prepared with either Ringer lactate or a combination of red blood cells and fresh frozen plasma. Each circuit operated at a flow of 3 L/min at 37°C for 24 hours. Remimazolam (1 mg) or midazolam (1 mg) was injected, creating four experimental groups. Blood samples for measurement of drug concentrations were collected at predefined time points. The percentage of drug remaining at 24 hours was comparable for remimazolam and midazolam (mean [standard deviation {SD}]: 3.7% [0.9] and 2.9% [0.5]) in Ringer lactate–perfused circuits and higher for midazolam compared with remimazolam (mean [SD] 52% [13] and 15% [2]) in blood-perfused circuits. The concentration of the remimazolam metabolite CNS7054 increased from 29 (8) ng/ml at 5 minutes to 173 (31) ng/ml at 24 hours. Drug sequestration of remimazolam in in-vitro ECMO circuit is comparable to midazolam in circuits perfused with Ringer lactate. In circuits perfused with blood, the percentage of drug remaining at 24 hours is lower for remimazolam.