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Compound-specific chlorine-isotope analysis (CSIA-Cl) of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (p,p′-DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethene (p,p′-DDE) in blubber from Baltic Grey seal (Halichoerus grypus) was performed in order to investigate if a kinetic isotope effect (KIE) could be observed concomitant to environmental degradation of DDT. The δ37Cl of p,p′-DDT and p,p′-DDE were −0.69±0.21‰ and −2.98±0.57‰ (1σ, n =3), respectively. Both samples were enriched relative to the hypothesized initial isotope composition (−4.34‰), thus indicating a composite KIE associated with the degradation mechanisms pertaining to DDT. An isotope fractionation factor for degradation of dichloromethane, from the literature, was adapted and modified for use in the calculation of DDT degradation. A subsequent simplified Rayleigh distillation model of the DDT chlorine-isotope composition yielded an estimated fraction (f) of 7±2% of released DDT presently remaining as undegraded compound in the environment. The consistency between the result of the Rayleigh model (f ∼7%) and the use of the DDT/(DDT+DDE) ratio as a measure of DDT degradation (∼10% undegraded DDT) suggests that the KIE of DDT degradation may be significant, and that the novel approach of CSIA-Cl may be a valuable tool for degradation/persistence studies of lipophilic organochlorines in the environment. [Copyright &y& Elsevier]