The equilibrium geometries, stability, and isomerisation of the ethynyl pseudohalides, HCC–X (where X = –NCO, –OCN, –CNO, –ONC), have been investigated by ab initio MP2 and CCSD(T), as well as by B3LYP density functional methods using the 6-311G(2d,2p) basis set. Minimum energy structures and their interconnecting transition states have been calculated, and possible isomerisation pathways are suggested. Calculations have predicted that three isomers, HCC–NCO, HCC–OCN, and HCC–CNO, are kinetically stable toward unimolecular isomerisation or dissociation at room temperature with the lowest kinetic energy barrier of 274.5, 200.3, and 261.6 kJ mol⁻¹, respectively (CCSD(T)//B3LYP), and other isomers are unstable. As a particular interest, the condensed phase behaviour of HCC–NCO, HCC–OCN, and HCC–CNO, viz. potential cycloaddition reactions, have been also studied at the B3LYP level. Theoretical calculation have indicated that these molecules are stable below room temperature in the condensed phase, but they dimerise and polymerise at elevated temperatures.