Planar metal–diamond–metal photoconductive devices have been fabricated from free standing large grain (20–30 &mgr;m) polycrystalline thin film diamond. An interdigitated electrode design with spacings of 20 &mgr;m was used to produce effective UV photodetecting devices at bias values in the range 0.1–10 V. A methane‐air treatment has been used to modify the structures such that unprecedented performance characteristics have been recorded (106higher response to 200 nm than visible wavelengths, <0.1 nA dark currents); spectral features similar to those observed in natural diamond crystals have been observed indicating that the treatment used led to near ideal electronic characteristics from polycrystalline material. ©1995 American Institute of Physics.