A procedure for the simultaneous and fast determination of methylmercury (MeHg⁺), inorganic mercury (Hg²⁺) and mono-, di- and tributyltin species (MBT, DBT, TBT) in water samples, as ethylated derivatives using a multicapillary GC column and MIP-AES detection, has been developed. The influences of the auxiliary gas pressure and the helium cavity vent flow in the MIP-AES response to mercury and tin species were evaluated and adjusted to obtain optimal detection conditions for both groups of organometallic compounds in the same chromatographic run. The proposed method was applied to the fast and simultaneous determination of mercury and tin species in water samples after a concentration step using solid-phase microextraction fibres.