We compared absolute values of regional tissue hemoglobin saturation (StO(2)), reproducibility, and dynamic range of four different instruments on the forearm of adults. The sensors were repositioned 10 times on each subject. Dynamic range was estimated by exercise with subsequent arterial occlusion. Mean StO(2) was 70.1% ± 6.7 with INVOS 5100, 69.4% ± 5.0 with NIRO 200 NX, 63.4% ± 4.5 with NIRO 300, and 60.8% ± 3.6 with OxyPrem. The corresponding reproducibility S(w) was 5.4% (CI 4.4-6.9), 4.4% (CI 3.5-5.2), 4.1% (CI 3.3-4.9), and 2.7% (CI 2.2-3.2), respectively. The dynamic ranges ΔStO(2) were 45.0%, 46.8%, 44.8%, and 27.8%, respectively. In conclusion, the three commercial NIRS instruments showed different absolute values, whereas reproducibility and dynamic range were quite similar.