Orujo is a waste product of the olive oil mill industry that still has significant oil content. Before extracting the remaining oil with hexane. the moisture of the wet waste product has to be reduced from approximately 65% to about 8% (wet basis). The high water content of orujo and its stickiness make the drying process difficult. The most common solution adopted by the industry is to use two rotary dryers in serie. In the first one, a mix of fresh and dried orujo with a moisture content of approximately 52-57% (w.b.) is introduced and leaves the dryer with a 25-35% (w.b.). There the main issue is not to control the outlet solid moisture but to treat as much solid as possible and to overcome serious operational problems. Currently the dryers are operated manually or, at the most, with a simple system to control the inlet gas temperature. We have developed, implemented and tested at an industrial dryer a control system based on PID controllers (applied to the first dryer) that minimise the operational problems and improves the production and the energetic efficiency. (C) 2004 Elsevier Ltd. All rights reserved.