Transparent prismatic crystals of Pb ₂(AsO ₂OH)Cl ₂ were collected in black metallurgical slag on the coast in the Punta Zeza area, 3 km south of the town of Lavrion, Greece. Analyses by energy-dispersive X-ray spectrometry (EDS) provided the following chemical composition: PbO 73.04, As ₂O ₃ 15.97, Cl 11.42, O=Cl-2.58, total 97.85 wt.%. The empirical formula calculated on the basis of As = 1 is H _x Pb _2.03(AsO ₃) _1.00Cl _1.995 (x = 0.96). The infrared spectrum of Pb ₂(AsO ₂OH)Cl ₂ has characteristic AsO ₃ ^3- bands at 707 and 594 cm ^-1, O-H stretching vibrations at 3310 and 2900 cm ^-1 and a band at 1107 cm ^-1 which is assigned to As-O-H bending vibrations. The structure, which is monoclinic P21/m, a = 6.4235(8), b = 5.5399(7), c = 9.321(1) Å, β = 90.767(2), V = 331.67(7) Å ³, R ₁ = 0.035, is identical to that of synthetic Pb ₂(AsO ₂OH)Cl ₂ and contains two symmetrically unique Pb sites and one As site. The crystal structure is based on [Pb ₂AsO ₂OH] ²⁺ double chains interconnected via weak Pb-Cl bonds to produce a three-dimensional framework which is closely related to that of Pb oxysalt minerals containing [O ₂Pb ₃] ²⁺ chains of oxocentred [OPb ₄] ⁶⁺ tetrahedra including mendipite, damaraite, rickturnerite and plumboselite.