The H–D exchange processes in MHn or MDn hydrides (M = As, Sb, Bi, n = 3; M = Ge, Sn, n = 4) taking place when they are in contact with H2O or D2O solution at different pH or pD values (interval of pH = [0,13]) have been investigated using gas chromatography–mass spectrometry (GC-MS). MHn or MDn compounds were injected into the headspace of reaction vials (4–12 ml) containing 1–2 ml of buffered solution maintained under stirring or shaking conditions. The isotopic composition of the gaseous phase hydrides/deuterides was determined at regular intervals in the range of time 0–15 min. The MHn or MDn compounds were synthesized in separate vials and their purity was checked separately before injection into the reaction vials. The mass spectra were deconvoluted in order to estimate the relative abundance of each species formed following the H–D exchange process (AsHnD3−n , SbHnD3−n, BiHnD3−n, n = 0–3; GeHnD4−n, SnHnD4−n, n = 0–4) and the relative abundance of H and D. In the investigated pH (or pD) interval arsanes and stibanes undergo H–D exchange in alkaline media for pH > 7. No H–D exchange was detected for the other hydrides, where the prevailing process is their decomposition in the aqueous phase. A reaction model, based on the formation of protonated or deprotonated intermediates is proposed for H–D exchange of MHn or MDn compounds placed in contact with H2O or D2O at different pH or pD values. The H–D exchange in the already formed hydrides can be source of the interference in mechanistic studies on hydride formation performed using labeled reagents; no H–D exchange was detected within the following pH intervals that can be considered free from interference: arsanes pH = [0,7), stibanes pH = [0,7), bismuthanes, germanes and stannanes pH = [0,13].