Analysis of stable metal isotopic ratios provides a novel tool to explore metal uptake and transfer mechanisms in soil-plant systems. In this study a pot experiment was conducted to investigate cadmium (Cd) isotope fractionation after repeated phytoextraction by the Cd and zinc (Zn) hyperaccumulator Sedum plumbizincicola with five successive crops in four highly Cd-contaminated agricultural soils. Two of the soils were paddy soils (S1 and S4) and the other two were upland soils (S2 and S3) collected from lead-zinc mining areas in Hunan province, central China. The Cd isotopic composition of all four soils was determined before and after five crops of phytoextraction, in the shoots of the first crop of S. plumbizincicola , and in the NH₄Ac-extractable Cd pool of soils S1 (pH 5.73) and S3 (pH 7.23) before and after repeated phytoextraction. In the initial soils before phytoextraction the Cd extracted by NH₄Ac was isotopically lighter than or showed no significant difference from soil total Cd (Δ^114/110Cd_{extract−soil} = -0.152 to 0.007 ‰). In addition, larger fractionation of light isotopes between the NH₄Ac-extractable Cd and total Cd in the paddy soil S1 (Δ^114/110Cd_{extract−soil} = -0.152‰) than the upland soil S3 (Δ^114/110Cd_{extract−soil} = 0.007‰) indicates that more Cd leaching from paddy soil than from upland soil preferentially removes heavy Cd isotopes. Cadmium in the shoots of S. plumbizincicola was significantly isotopically heavier than Cd in soils (Δ^114/110Cd_shoot−soil = 0.022 to 0.132‰) and Cd in the soils after phytoextraction was significantly isotopically lighter than Cd in soils before phytoextraction (Δ^114/110Cd_{soil: P5−soil: P0} = -0.146 to -0.032‰). This demonstrates that S. plumbizincicola favours heavy Cd isotopes. However, there was no significant isotope fractionation between Cd extracted by NH₄Ac and Cd in shoots, implying that NH₄Ac-extractable Cd constitutes the plant-available Cd pool to S. plumbizincicola. After phytoextraction, NH₄Ac-extractable Cd isotopes significantly fractionated with Δ^114/110Cd_{extract: P5−extract: P0} values of 0.074 to 0.180 ‰. This may result from the replenishment of heavier isotopes from other soil Cd pools. Based on a previous observation that S. plumbizincicola was enriched in light Zn isotopes, this suggests that S. plumbizincicola may have a Cd uptake and transfer mechanism different from Zn. Sedum plumbizincicola may have a special channel for Cd transportation which leads to an enrichment of heavy Cd isotopes in the aboveground parts. This is inconsistent with the supposition that plants take up Cd via their Zn transport systems.