The crystal structures of zincroselite and gaitite, two polymorphs of Ca 2 Zn[AsO 4 ] 2 ·2H 2 O from Tsumeb (Namibia), were determined from single-crystal X-ray intensity data. The type crystal of zincroselite, monoclinic, P 2 1 / c , a = 5.827(1), b = 12.899(3), c = 5.646(1) A and β = 107.69(3)°, Z = 2, was refined with 935 unique reflections to R 1 = 2.61%. The mean bond distances are = 1.690 A, 2 O)> = 2.123 A and 2 O)> = 2.526 A. Gaitite, P 1, a = 5.899(1), b = 6.978(1) and c = 5.755(1) A, α = 97.41(3)°, β = 109.08(3)° and γ= 108.09(3)°, Z = 1, was refined with 914 unique reflections to R 1 = 1.77%. The mean bond distances are = 1.687 A, 2 O)> = 2.102 A and 2 O)> = 2.525 A. For both structures, the positions of all H atoms were isotropically refined and the hydrogen-bonding arrangement identified. The common structural unit of zincroselite and gaitite are [Ca 2 Zn(H 2 O) 2 O 8 (AsO 4 ) 2 ] 16- rods parallel to the c axis. The polymorphism of Ca 2 Zn[AsO 4 ] 2 ·2H 2 O referred to the glide plane c can easily be visualized with different rod-packings. Zincroselite and gaitite belong to the roselite and collinsite structure type, respectively.