Waste citrus peels can be used as source of several bioactive compounds. Among these, D-limonene is the major constituent in lemon essential oil and is of great interest in several fields. With the aim of optimizing D-limonene extraction from lemon peels after citrus processing, a non-conventional solvent extraction was studied. Hexane was used as solvent at high pressure and temperature and influence of extraction time, temperature-pressure and matrix/solvent ratio (M) on yield of D-limonene was analyzed by a statistical approach applied to a three-level full factorial design (3(3)). The highest yield was reached when extraction was performed with M = 1:15 at 150 degrees C for 30 min (3.56%). Furthermore, a response surface methodology (RSM) was used and experimental results were fitted by a second-order polynomial equation. The effects of drying pre-treatment and particle size were also evaluated in this work. The effectiveness of this innovative method in extraction of citrus essential oils -and, in particular, of D-limonene -has been evaluated and compared to conventional Soxhlet extraction. High pressure - high temperature extraction (HPTE) of D-limonene from lemon peels was better than Soxhlet extraction even with low matrix/solvent extraction (1:4) in terms of energy saving (0.6 kW h vs 2.5 kW h), extraction time (30 min vs 4 h) and product yield (2.97% vs 0.95%). (C) 2014 Elsevier B.V. All rights reserved.