Isotope ratio measurements of organometal(loid) species are of interest in order to determine isotope fractionation of the heavy metal(loid)s which can elucidate their pathways in the environment. We describe isotope ratio measurements of organoantimony using gas chromatography coupled to ICP-MS. We studied the influence of various parameters on the isotope ratio determination of the transient signal from trimethylstibine, using capillary gas chromatography coupled to both multicollector and time of flight ICP-MS with a simultaneous aspiration of aqueous samples creating wet plasma conditions. The influence of different mathematical models on the precision of the isotope ratio determination of transient signals was studied. The trapezium integration of full or half peak gave precisions of 0.08% and 0.02% using the MC-ICP-MS, whereas the ICP-TOF-MS gave values of around 0.2%. The multicollector ICP-MS used as a detector showed a chromatographic isotope fractionation of trimethylstibine. (CH3)3123Sb eluted earlier than (CH3)3121Sb, with a shift of the 123/121 ratio of about 2%. Two different approaches were attempted to correct instrumental mass bias effects in order to descriminate natural isotope fractionation. First, we introduced Cd and Sb in aqueous solution simultaneously with the GC carrier and ICP Ar gas flow. Secondly, we used a standard-sample bracketing approach using a gas standard introduced via the GC. Real samples from a laboratory sewage sludge fermentor were measured and isotope fractionation of the biologically produced trimethylstibine was observed (δ
123Sb +10 and +19‰). The gas samples were analysed for timethylstibine co-eluting with volatile organic compounds using GC-MS. Chloroform was found to co-elute, but had no significant influence on the isotope ratio of antimony in timethylstibine analysed by GC-ICP-MS. Hence, there would be a strong indication that isotope fractionation takes place when antimony is methylated by anaerobic bacteria.