Objective. The objective of our study was to describe the results from human experiments during normoxia that demonstrate the effect of pulsating arteries on the measured arterial oxygen saturation (Spo2) using a reflectance pulse oximeter sensor.Methods. In 6 healthy adults and 7 healthy neonates, a Nellcor reflection sensor (FS-10 oxisensor, Nellcor, Inc., Pleasanton, CA) was placed in three different positions: (1) on the forehead, (2) on the temporal area, with the photodiode placed over the superficial temporal artery, and (3) on the temporal area, with the light-emitting diodes (LEDs) placed over the superficial temporal artery.Results. Placement of the sensor in position 2 resulted in a significantly lower Spo2 reading, compared to sensor position 1: 5.8% (p<0.01) lower for adults and 7.5% (p<0.01) lower for neonates. Placement of the sensor in position 3 resulted in significantly larger plethysmographic signals, compared to sensor position 1; but, the Spo2 readings were alike.Conclusions. Pulsating arteries can affect the reliability of reflection pulse oximetry. Depending on the position of the sensor, a falsely low Spo2 value can be observed.