Different damage effects are observed in KDP and ADP when irradiated with light and heavy ion beams. Color centres stable up to 240 K are observed in both compounds irradiated with beams of 22 MeV2H+and fluences below 1 × 1016ions/cm2; a tentative mechanism for their generation is given. For fluences between 1 × 1016and 2 × 1016ions/cm2no further damage is observed in KDP; but ADP crystals show a light color stable up to the melting point and for higher fluences they appear white and opaque: X-ray diffraction patterns show no amorphization; a broad band due to electronic excitations, besides the characteristic Raman spectra is observed. No spontaneous cleavage was ever observed.