Various stages of an analytical method for high-precision cadmium (Cd) isotope ratio measurements by MC-ICP-MS (sample preparation, matrix separation, instrumental analysis and data evaluation) were critically evaluated and optimized for the processing of carbon-rich environmental samples. Overall reproducibility of the method was assessed by replicate preparation and Cd isotope ratio measurements in various environmental matrices (soil, sediment, Fe-Mn nodules, sludge, kidney, liver, leaves) and was found to be better than 0.1‰ (2σ for δ114Cd/110Cd) for the majority of samples. Cd isotope ratio data for several commercially-available reference materials are presented and compared with previously published results where available. The method was used in a pilot study focusing on the assessment of factors affecting Cd isotope composition in tree leaves. A summary of results obtained for a large number (n > 80) of birch (Betula pubescenes) leaves collected from different locations in Sweden and through the entire growing season is presented and potential reasons for observed variability in Cd isotope composition are discussed. Seasonal dynamics of element concentrations and isotope compositions in leaves were also compared for Os, Pb, Zn and Cd.