Methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) significantly contribute to stratospheric ozone depletion. The atmospheric budgets of both compounds are unbalanced with known degradation processes outweighing known emissions. Stable isotope analysis may be capable to identify and quantify emissions and to achieve a balanced budget. Degradation processes do, however, cause isotope fractionation in methyl halides after emission and hence knowledge about these processes is a crucial prerequisite for any isotopic mass balance approach. In the current study, triple-element isotope analysis (²H, ¹³C, ³⁷Cl/⁸¹Br) was applied to investigate the two main abiotic degradation processes of methyl halides (CH₃X) in fresh and seawater: hydrolysis and halide exchange. For CH₃Br, nucleophilic attack by both H₂O and Cl^– caused significant primary carbon and bromine isotope effects accompanied by a secondary inverse hydrogen isotope effect. For CH₃Cl only nucleophilic substitution by H₂O was observed at significant rates causing large primary carbon and chlorine isotope effects and a secondary inverse hydrogen isotope effect. Observed dual-element isotope ratios differed slightly from literature values for microbial degradation in water and hugely from radical reactions in the troposphere. This bodes well for successfully distinguishing and quantifying degradation processes in atmospheric methyl halides using triple-element isotope analysis.