A method and apparatus for non-contact determination of the moisture content in moving or stationary mass of fibrous products such as wood is provided. The method comprises irradiation of a substance with infra-red radiation which is intense enough to introduce microstructural modifications of the substance surface. The moisture content of the substance is determined by analyzing density of these surface modifications. It has been found, that an optical beam may be used in order to quantify the density of the surface modifications. The surface is scanned with the optical beam two (2) times. The first scanning is done prior to the infra-red irradiation; the second scanning is done following the infra-red irradiation. The invented method allows for non-contact, rapid and continuous in-line measurements of the moisture content.