In principle, the measurement of the time-resolved diffuse reflectance, as interpreted on the basis of the diffusion theory, allows us to determine the optical properties (absorption coefficient mu(a) and transport scattering coefficient mu's) of biological materials noninvasively. A system for time-resolved reflectance measurements and its performances are described in detail. An extensive experimental work and a computer simulation characterized the influence of mu(a), mu's, and observation distance on the measurement, and allowed to determine the ranges of mu(a) and mu's which can be measured reliably (and which include most of the values of interest in biomedical applications). Moreover, on the basis of the experimental and computer work, the optimal experimental parameters can be determined in various situations. The results presented are general and can be extended to different systems of measurement.