An increased interest in high precision Cu isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has developed recently for various natural geologic systems and environmental applications, these typically contain high concentrations of sulfur, particularly in the form of sulfate (SO₄^2-) and sulfide (S). For example, Cu, Fe, and Zn concentrations in acid mine drainage (AMD) can range from 100??g/L to greater than 50mg/L with sulfur species concentrations reaching greater than 1000mg/L. Routine separation of Cu, Fe and Zn from AMD, Cu-sulfide minerals and other geological matrices usually incorporates single anion exchange resin column chromatography for metal separation. During chromatographic separation, variable breakthrough of SO₄^2- during anion exchange resin column chromatography into the Cu fractions was observed as a function of the initial sulfur to Cu ratio, column properties, and the sample matrix. SO₄^2- present in the Cu fraction can form a polyatomic ³²S-¹⁴N-¹⁶O-¹H species causing a direct mass interference with ⁶³Cu and producing artificially light ??⁶⁵Cu values. Here we report the extent of the mass interference caused by SO₄^2- breakthrough when measuring ??⁶⁵Cu on natural samples and NIST SRM 976 Cu isotope spiked with SO₄^2- after both single anion column chromatography and double anion column chromatography. A set of five 100??g/L Cu SRM 976 samples spiked with 500mg/L SO₄^2- resulted in an average ??⁶⁵Cu of -3.50?????5.42??? following single anion column separation with variable SO₄^2- breakthrough but an average concentration of 770??g/L. Following double anion column separation, the average SO₄^2-concentration of 13??g/L resulted in better precision and accuracy for the measured ??⁶⁵Cu value of 0.01?????0.02??? relative to the expected 0??? for SRM 976. We conclude that attention to SO₄^2- breakthrough on sulfur-rich samples is necessary for accurate and precise measurements of ??⁶⁵Cu and may require the use of a double ion exchange column procedure. ?? 2010.