A combination of neutron activation and gamma-ray coincidence counting technique is used to determine the concentration of both long-lived fission produced129I and natural127I in environmental samples. The neutron reactions used for the activation of the iodine isotopes are129I(n, )130I and127I(n, 2n)126I. Nuclear interferences in the activation analysis of129I and127I can be caused by production of130I or126I from other constituents of the materials to be irradiated, i.e. Te, Cs and U impurities and from the125I tracer used for chemical yield determination. Chemical interferences can be caused by129I and127I impurities in the reagents used in the pre-irradiation separation of iodine. The activated charcoals used as iodine absorbers were carefully cleaned. Different chemical forms of added125I tracer and129I and127I constituents of the samples can cause different behaviour of125I tracer and sample iodine isotopes during pre-irradiation separation of iodine. The magnitude of the nuclear and chemical interferences has been determined. Procedures have been developed to prevent or control possible interferences in low-level129I and127I activation analysis. For quality control a number of biological and environmental standard samples were analyzed for127I and129I concentrations.