Radiation detectors based on diamond are attractive for radiation protection applications in space and aviation due to their radiation hardness, large breakdown voltage, fast signal collection, and low noise, thus representing a valid alternative to silicon-based detector devices. In this paper, we study the tissue equivalence of diamond in proton and $\alpha $ particle radiation fields typical of galactic cosmic rays and solar particle events. We determine a method to convert microdosimetric response from diamond to water by means of Geant4 simulations. Results are presented showing that a simple geometrical scaling factor (∼ 0.32) of linear dimensions is adequate to convert experimentally obtained microdosimetric energy deposition spectra in diamond to equivalent microdosimetric energy deposition spectra in water. © 2013 IEEE.