Abstract The crystal structure of Pr(III) hydroxide perchlorate monohydrate, Pr 2 (OH) 3 H 2 O(ClO 4 ) 3 , has been determined from single-crystal X-ray diffraction data. The unit cell (monoclinic, C 2/ c , a =1576.1(4) pm, b =961.8(3) pm, c =1033.2(3) pm, β =119.97(3)°, Z =4, R =0.023, R w =0.022) contains eight Pr 3+ ions in ninefold oxygen coordination. Seven O 2− ions from perchlorate anions and two O 2− ions from hydroxide anions or crystal water form a distorted tricapped trigonal prism surrounding Pr 3+ . Two crystallographically and functionally different ClO 4 − anions bridge two Pr 3+ cations to form anionic centrosymmetric dimers with the composition ( [Pr(OH) 3 2 (H 2 O) 1 2 (ClO 4 ) 4 ] 2 ) 5− . Further connection via “free” coordination positions of perchlorate oxygen leads to one-dimensional infinite chains which run parallel to [001]. These are stacked in the [010] direction in the fashion of a hexagonal closest packing of rods that are further connected via perchlorate oxygen assisted by (weak) hydrogen bonds to form a three-dimensional network.