A novel electro-optical sensing method for measuring the displacements of a micromachined accelerometer proof mass is described. By using on-chip integrated photodiodes, and by modulating the illumination, accelerations at very low frequencies are detected. A 10μg/√Hz noise equivalent acceleration (NEA) is predicted. Characterisation of the CMOS read-out circuits and on-chip integrated sensing photodiodes is presented. Preliminary measurements on a prototype device are reported.