The aim of this study was to characterise the simplest compound models with a selenium–oxygen, selenium–sulphur and selenium–selenium bond as the SeXH and HSeXH isomers (X = O,S,Se). One of the main aspects of this investigation was to provide a description on the isomerisation pathways involving 2 [H,Se,X] and 1 [2H,Se,X] potential energy surfaces calculated at the CCSD(T)/CBS//MP2/cc-pVTZ level. The energy difference was 13 kcal mol −1 between hydroxyselenide (SeOH) and oxoselenium (HSeO), while a gap of 3 kcal mol −1 was predicted between thiol-selenide (SeSH) and selenol-sulphide (HSeS). The SeOH→HSeO unimolecular rearrangement showed a barrier energy of 44.6 kcal mol −1 , decreasing almost two times in sulphur and selenium analogous reactions. In addition, hydroxyselenide (HSeOH), thioselenenic acid (HSeSH) and diselane (HSeSeH) were the global minimum configurations in the ground state, while the energy differences among the other isomers were close to 30 kcal mol −1 . The HSeXH→H 2 SeX and HSeXH→SeXH 2 isomerisations showed barrier energies ranging from 40 to 65 kcal mol −1 , while these reverse routes presented heights that were three times smaller. The kinetic rate constant of each 1,2-H shift reaction was performed here as well as an analysis of the selenium-chalcogen bonds using natural bond orbital and bond order index methodologies.