The oxygenation velocity of Hb was studied by using the conventional rapid flow method. Specifically, it was compared in 4 and multiple jet type mixing chambers. A steeper rise of So2 was observed in the latter multiple jet chamber at an early stage of reaction, probably due to the enhanced efficiency of mixing. The oxygenation velocity was expressed in this study by a rate factor which was calculated by dividing the O2uptake by the difference between the back pressure and the Po1 of the surrounding medium. The former pressure was read from the O2 dissociation curve of Hb by referring to the So1. This factor was not lineally proportional to the undersaturation, but went up to the 1.6th power of it.