In the framework of a certification campaign organized by BCR (Bureau Communautaire de Reference, Commission of the European Communities, Brussels) molybdenum was determined in a sea water candidate reference material (BCR CRM 403) using inductively coupled plasma-mass spectrometry. The determination was hampered by both non-spectral (signal suppression) and spectral interferences. Ten-fold dilution of the sea water and the use of a carefully selected internal standard allowed accurate correction for the signal suppression. Spectral interferences on Mo nuclides could mainly be attributed to BrO+ and BrOH+ ions. At the level of spectral overlap encountered, these interferences could be corrected for with sufficient accuracy by matrix matching of the blank for Br or by application of a mathematical correction method involving resolvation of a pair of simultaneous equations. Results obtained after application of anion exchange to separate Br from Mo confirmed the results obtained using the correction methods, proving the validity of the latter. A good agreement is established by comparison of the ICP-MS result with those obtained by other techniques in other laboratories.