Deuterium resonance investigations of KD3(SeO3)2 single crystals have been performed near the phase transition temperature T
C. There are two types of deuterium bonds in these crystals with different behaviors at this phase transition. Our experimental results show that there are significant changes in the D spinlattice relaxation time T
1 at T
C; the abrupt decrease in T
1 near T
C can be explained by the critical slowing down of an overdamped soft pseudospin-type deuteron mode. Further, the ordering of the O(2)…D… O(2) bonds is affected by the phase transition, whereas the ordering of the O(1)-D… O(3) bonds is unaffected. The D NMR measurements also show that the D(2) deuteron disordering above T
C is dynamic and not static.