We have designed and constructed a near-infrared spectrometer for the non-invasive optical study of biological tissue. This instrument works in the frequency-domain and employs multiple source-detector distances to recover the absorption coefficient (μa) and the reduced scattering coefficient (ixj) of tissue. The light sources are eight light emitting diodes (LEDs) whose intensities are modulated at a frequency of 120 MHz. Four LEDs emit light at a peak wavelength of 715 nm (λ1), while the other four LEDs emit at a peak wavelength of 850 nm (X2). From the frequency-domain raw data of phase, DC intensity, and AC amplitude obtained from each one of the eight light sources, which are located at different distances from the detector fiber, we calculate μa and μs at the two wavelengths λ1 and λ2. The concentrations of oxy- and deoxy-hemoglobin, and hence hemoglobin saturation, are then derived from the known extinction coefficients of oxy- and deoxy-hemoglobin at λ1 and λ2. The statistical error in the measurement of the optical coefficients due to instrument noise is about 1-2%. The accuracy in the determination of the absolute value of the optical coefficients is within 10-20%. Preliminary results obtained in vivo on the forearm of a volunteer during an ischemia measurement protocol are presented.