The microwave steam diffusion (MSDf) apparatus for extraction of essential oils from orange peel (by-products) was studied. MSDf has been compared with conventional steam diffusion (SDf). A response surface methodology (RSM) was realised to investigate the influence of process variables by a central composite design (CCD) approach. The statistical analysis revealed that the optimal conditions for the extraction of orange essential oil were a steam mass flow rate of 25 g min À1 and a microwave power of 200 W. Cytohistological investigations using light microscopy provided evidences for rapid and pronounced cell and tissue alterations of oil glands with MSDf in comparison with SDf. The essential oils extracted by MSDf for 12 min were quantitatively (yield) and qualitatively (aromatic profile) similar to those obtained by SDf for 40 min. Extraction of essential oils from orange peel with MSDf was better than SDf in terms of energy saving, cleanliness and reduced waste water.