The interpretation of cerebral tissue oxygen saturation values (StO2) in clinical settings is currently complicated by the use of different near-infrared spectrophotometry (NIRS) devices producing different StO2 values for the same oxygenation due to differences in the algorithms and technical aspects. The aim was to investigate the effect of changes in scattering and absorption on the StO2 of different NIRS devices in a liquid optical phantom. We compared three continuous-wave (CW) with a frequency domain (FD) NIRS device. Responsiveness to oxygenation changes was only slightly altered by different intralipid (IL) concentrations. However, alterations in haematocrit (htc) showed a strong effect: increased htc led to a 20-35% increased response of all CW devices compared to the FD device, probably due to differences in algorithms regarding the water concentration.