We described a low-cost, frequency-domain photon migration spectroscopy instrument by using in-phase and quadrature demodulation technique in a 140-MHz homodyne system. A time constant of 1 ms was obtained. A frequency-division multiplexing technique was used for multiposition or multiwavelength applications, such as for tissue oximetry. By conducting several experiments on tissue/blood models, we show that the optical properties of tissue can be determined with this system by either using multiple source-detector separations at accuracy better than 8% or using single source-detector separation. The accuracy with the latter method can be improved to be 10% or better by choosing a proper calibration sample. A frequency-sweep system (30 to 700 MHz) was also implemented; the phase and amplitude measured on a tissuelike solution at 780 nm agreed to their theoretical values within 5% and 10% accuracy, respectively. © 1997 Society of Photo-Optical Instrumentation Engineers.