Abstract Lead and copper(I) chloride arsenite was found in the ancient metallurgic slag from the Vrissaki area, Lavrion district, Attikí Peninsula, Greece. Its chemical composition corresponds to the idealized formula Pb₆Cu⁺(AsO₃)₂Cl₇. The IR spectrum shows the presence of A s O 3 3 − $AsO_3^{3 - }$ anions and only a trace amount of O–H bonds. The crystal structure was solved by direct methods and refined to R(F) = 0.0304 based on 1778 unique reflections with I > 2σ(I). The compound is trigonal (rhombohedral), R3̅, a = 9.8691(2), c = 34.2028(13) Å, V = 2885.01(14) ų, Z = 6. Two crystallographically non-equivalent As³⁺ cations occupy apexes of the AsO₃ pyramids. Cu⁺ cation occupies an apex of the CuCl₃ pyramid. The group [Cl₃Cu–AsO₃] is arranged along the c axis. Pb cations occupy two sites with seven- and eight-fold coordination. The crystal-chemical formula of the compound is Pb₆(Cu⁺Cl₃)(As³⁺O₃)₂Cl₄.