Abstract: An oximeter apparatus especially adapted to reflectance oximetry includes a sensor having a red LED, an infrared LED and a photosensor resiliently mounted in a carrier to provide constant linear pressure characteristics. The LED's and photodetector are fixed to a flexible circuit board. The d.c. components of the reflectance signals are fed back to control the brightness of the LED's A.c. component signals from the photodetector are rejected in the determination of oxygen saturation when the red and infrared a.c. components differ by more than a determined amount. The difference between the maximum and minimum values of each pulse is employed in the determination of oxygen saturation, with two measurements being taken for each pulse.