A modified automated on-line hyphenated system for simultaneous inorganic ionic mercury (Hg(2+)) and monomethylmercury (MeHg(+)) analysis by hydride generation (HG) or ethylation (Eth), cryofocussing, gas chromatography (GC) separation and atomic fluorescence spectrometry (AFS) detection has been improved. Both derivatization methods are investigated with respect to the chromatographic and analytical performances. They can be both affected by interferences when the AFS detection system is used. Water vapor removal using a soda lime moisture trap improves significantly the chromatographic performances, the reproducibility and the detection limits for Hg(2+) and MeHg(+) analyzed with both methods. For ethylation (Eth) derivatization, a scattering interference generated from low-quality ethylation reagent has also been eliminated. For HG, improved detection limits are 0.13ngl(-1) and 0.01ngl(-1) for Hg(2+) and MeHg(+), respectively (0.1l water sample), and reproducibility are 5% for Hg(2+) (20ngl(-1)) and MeHg(+) (5ngl(-1)). Improved detection limits for Eth are 0.22ngg(-1) for Hg(2+) and 0.02ngg(-1) for MeHg(+) (1g dry sediment sample) and the reproducibility are 5-6% for Hg(2+) and MeHg(+) (1-2ngg(-1)).