We determine temperature effect on the absorption and reduced scattering coefficients and of human forearm skin. Optical and thermal simulation data suggest that and are determined within a temperature-controlled depth of ≈2 mm. Cutaneous change linearly with temperature. Change in was complex and irreversible above body normal temperatures. Light penetration depth (δ) in skin increased on cooling, with considerable person-to-person variations. We attribute the effect of temperature on to change in refractive index mismatch, and its effect on to perfusion changes. The reversible temperature effect on was maintained during more than 90 min. contact between skin and the measuring probe, where temperature was modulated between 38 and 22 °C for multiple cycles While temperature modulated instantaneously and reversibly, exhibited slower response time and consistent drift. There was a statistically significant upward drift in and a mostly downward drift in over the contact period. The drift in temperature-induced fractional change in was less statistically significant than the drift in values determined under temperature modulation conditions may have less nonspecific drift than which may have significance for noninvasive determination of analytes in human tissue. © 2003 Society of Photo-Optical Instrumentation Engineers.