A MEMS device, for example a capacitive microphone, comprises a flexible membrane 11 that is free to move in response to pressure differences generated by sound waves. A first electrode 13 is mechanically coupled to the flexible membrane 11 , and together form a first capacitive plate of the capacitive microphone device. A second electrode 23 is mechanically coupled to a generally rigid structural layer or back-plate 14, which together form a second capacitive plate of the capacitive microphone device. The capacitive microphone is formed on a substrate 1 , for example a silicon wafer. A back-volume 33 is provided below the membrane 11 , and is formed using a 'back-etch' through the substrate 1. A first cavity 9 is located directly below the membrane 11 , and is formed using a first sacrificial layer during the fabrication process.; Interposed between the first and second electrodes 13 and 23 is a second cavity 17, which is formed using a second sacrificial layer during the fabrication process. A plurality of bleed holes 15 connect the first cavity 9 and the second cavity 17. Acoustic holes 31 are arranged in the back-plate 14 so as to allow free movement of air molecules, such that the sound waves can enter the second cavity 17. The first and second cavities 9 and 17 in association with the back-volume 33 allow the membrane 11 to move in response to the sound waves entering via the acoustic holes 31 in the back-plate 14. The provision of first and second sacrifjciaj layers has the advantage of protecting the membrane during manufacture, and disassociating the back etch process from the definition of the membrane. The bleed holes 15 aid with the removal of the first and second sacrificial layers.; The bleed holes 15 also contribute to the operating characteristics of the microphone.