In this paper the results of an experimental study of heat transport peculiarities in a composite system “porous matrix - liquid” are presented where porous silicon with different values of porosity was used as the matrix. For evaluation of the thermal conductivity, a photoacoustic technique in a classical configuration with a gas-microphone registration was used. As a result of the experimental research, the increase of the thermal conductivity (up to two times) of the composite system “porous silicon - viscous liquid” in comparison with porous silicon has been determined. It was shown that such an increase is caused mainly by improved thermal contact between nanocrystallites of the PSi matrix as the result of the filling of second-order pores.