The method consists of measuring the perturbation provoked by a small volume of the diffusive medium on light propagating through a medium of known optical properties. The absorption and the reduced scattering coefficients of the medium are retrieved from multidistance continuous-wave measurements of transmittance. The inversion procedure is based on the solution of the diffusion equation obtained with a perturbative approach. The method has been validated with Monte Carlo results. Examples of experimental results are reported.