The normal vibration modes of deuterated potassium dihydrogen phosphate (DKDP) crystals were analyzed basing on the space group theory. The lattice vibration spectra of DKDP, which arise mainly from internal vibrations of P-O tetrahedrons, were recorded by a micro-Raman spectrophotometer system. The four characteristic internal vibration modes of the D2PO4 anion in KD2PO4 are assigned as 880 cm(-1) (nu(1), asymmetrical P(OD)(2) stretching vibration), 356 cm(-1) (nu(2), in-plane P(OD)(2) bending vibration), 512/540 cm(-1) (nu(3), PO2 bending vibration) and 967 cm(-1) (nu(4), in-plane PO2 symmetrical stretching vibration). Excellent agreement has been obtained between the theoretical studies and experimental investigations. (C) 2004 Elsevier B.V. All rights reserved.