A high-performance liquid chromatography (HPLC) system with a C₁₈ column and an aqueous phase eluent (0.08% ammonium acetate and 0.02% L-cysteine) was directly connected to an inductively coupled plasma mass spectrometer (ICP-MS). HPLC-ICP-MS was used to study the abiotic formation of methylmercury, CH₃Hg⁺, from inorganic mercury, Hg²⁺, as well as demethylation of CH₃Hg⁺ to Hg²⁺ in biological tissues during treatment with tetramethylammonium hydroxide (TMAH) followed by pH adjustment with citric or acetic acid. Enriched isotope standards from CH₃¹⁹⁸Hg⁺ and ²⁰¹Hg²⁺ were added to the samples to monitor species transformation and to apply species-specific isotope dilution (SSID) calibration. Depending on the type of sample matrix, up to 11.5% of added Hg²⁺ was methylated and up to 6.26% CH₃Hg⁺ was demethylated to Hg²⁺. Methylation of Hg²⁺ probably takes place mainly during and after pH adjustment and it decreases after prolonged TMAH treatment. To minimize abiotic methylation, it is therefore recommended to proceed with pH adjustment after samples have been treated with TMAH for 24 h. There is no significant difference in the degree of methylation using citric or acetic acid.