•We present the main characteristics of ISA accelerometer (Italian Spring Accelerometer), a tri-axial electro-mechanical accelerometer for space use, to be embarked on-board the BepiColombo mission towards the Mercury planet (2017).•We depict the issue of the zero-g positioning for the accelerometer, i.e. the measure of the sensing mass rest position when the sensor is in free-fall conditions (named briefly zero-g), condition that would be observed in orbit.•We present an experimental procedure and a zero-g facility to measure the zero-g positioning verified on DM and EQM models of ISA.•We show some results of tests carried out on an EQM model of ISA BepiColombo is a joint ESA/JAXA mission devoted to study the planet Mercury through two orbiters (MPO and MMO) to be launched in 2017. Among the main objectives, the European MPO aims at measuring the gravitational field of Mercury with unprecedented accuracy, through a careful reconstruction of the spacecraft orbit. The use of a very high sensitive tri-axial accelerometer, named ISA, will improve the orbital determination through an accurate sensing of the non-gravitational accelerations experienced by the spacecraft, mainly due to the solar radiation. The development of the three accelerometers, aluminium-made and constituted by electro-mechanical transducers, faced different issues to reach the required performance. One of the most relevant topic was the zero-g positioning, i.e. the measure of the sensing mass rest position when the sensor is in free-fall conditions (named briefly zero-g), condition that would be observed in orbit. The sensor offers the best performance when the sensing mass operates around this equilibrium position. Therefore this position needs to be identified and the mass has to be located there to guarantee the requirements imposed by the measurement of the non-gravitational accelerations. The group of Experimental Gravitation at IAPS/INAF in Rome developed an experimental procedure and a set-up to measure the zero-g position, carrying out a measurement campaign. This procedure will be applied to measure the zero-g positioning of ISA flight models.