In-line venous saturation monitoring is a useful method of quality management during extracorporeal circulation (ECC). The Bentley SM-0200 OxySAT meter was tested with and without the use of the recommended cuvette. The study consisted of an in vitro part, where extremely low saturations could be safely measured, and an in vivo part, carried out under routine ECC conditions. The results show that the OxySAT may be used with or without a cuvette, and that it needs calibration for optimal accuracy in either case.