Abstract: A disc-type refiner wherein the means incorporating the refining surfaces are positioned in opposed, relatively adjacent, closely spaced relation and one thereof is formed to include at least one passageway through which steam created in the refining of material between the refining surfaces may be simply and quickly vented. A duct means is provided to receive and carry steam from the passageway to the exterior of the refiner housing, to facilitate use or disposal as and where desired. Means are included to control the amount of steam being vented and to maintain the venting arrangement on adjustment of the spacing between the refining surfaces. In the preferred embodiment illustrated, the invention features are incorporated in a double revolving disc refiner which, by reason of the improvements of the invention, may be operated under either atmospheric or pressurized conditions. In either case considerable benefit is reflected in the refining procedure and the quality of its end product.