Multi-collector ICP – mass spectrometry (MC-ICPMS) is an extremely powerful technique for the determination of isotope ratios. Except for H, C, N and O and the noble gases, the isotopic composition of nearly all elements can be determined via MC-ICPMS. Until a couple of years ago, only thermal ionisation MS (TIMS) provided sufficient precision to study the often extremely small natural or induced variations in the isotopic composition of an element. Methods are developed to determine natural abundances of metalisotopes in plant and fungal material.