A novel multianode electron detector chip (LEDA512) has been designed, fabricated and tested using a 0.7 μm complementary metal-oxide-semiconductor technology. This chip, mounted behind a stack of two microchannel plates, is used in a positive ion mass spectrometer selected by the European Space Agency for the ROSETTA cometary mission. Two functions are integrated on the chip: a dual row of 512 anodes, each having an area of 22 μm X 8 mm for the collection of electrons, and two separate anode multiplexer and charge amplifier units. For redundancy reasons the chip has been designed such that the two anode rows with their associated electronics are identical and may be used independently with separate clocks and power supplies. Tests have confirmed that the LEDA512/microchannel plate combination is working properly as an ion detector and that excellent noise and response may be obtained in combination with the mass spectrometer.