In order to clarify the mechanism of hydride formation, the isotopic composition of arsine, stibine, bismuthine, germane, stannane and hydrogen selenide formed by derivatization with either NaBD4 (TDB) or NaBH4 (THB) with inorganic As(III), Sb(III), Bi(III), Ge(IV), Sn(IV) and Se(IV) in aqueous reaction media and under various reaction conditions was determined. Batch hydride generation and gas chromatography–mass spectrometry (GC–MS) were employed. The analyte, present in 0.5–5 ml of acid solution (0.1–10 M in HCl or HNO3 or HClO4) was derivatized with 1 ml of 0.25–0.5 M TDB / THB in 0.1 M NaOH solution. For TDB derivatization in H2O reaction media, almost pure BiD3 and SbD3 were always obtained for Bi(III) and Sb(III). Nearly pure AsD3 could be obtained only under some reaction conditions. In general, for As(III), the isotopic composition of the arsines depends strongly on reaction conditions and included all possible AsHnD3−n from almost pure AsD3 to almost pure AsH3. For Ge(IV) and Sn(IV), the isotopic composition of generated GeHnD4−n and SnHnD4−n depends on reaction conditions, but pure GeD4 and SnD4 could never be obtained. Pure H2Se was obtained in all cases, independent of reaction conditions. The occurrence of side reactions involving D–H exchange in TBD during its hydrolysis and before the derivatization step, as well as on recently formed hydrides following derivatization was investigated. D–H exchange in TDB during acid hydrolysis appears to occur to a limited extent. Amongst the hydrides, H2Se undergoes H–D exchange whereas germane and stannane do not exchange at all. Arsine undergoes D–H exchange at elevated acidities (pH < 0) whereas stibine and bismuthine do not exchange significantly during the generation and stripping steps. A reaction model for hydride generation is proposed accounting for primary reactions giving rise to hydride formation through reaction intermediates, as well as side reactions involving D–H exchange and decomposition of reactive hydroboron species, reaction intermediates and final products. Hydrides are formed by direct hydrogen transfer from boron to the analyte atom, most likely through concerted mechanisms taking place via reaction intermediates.