The interstellar presence of molecules with a peptide moiety, -C(O) -NH -, is suggestive of an extraterrestrial origin for life on Earth. Acetamide, H 3 CC(O)NH 2 , is not only one of the most abundant organic molecules present in the neighbourhood of Sagittarius B2 but has also been found in comets. It has been postulated to be the key precursor to more complex organic species although there are currently no obvious routes that would explain its formation in the gas-phase. The very recent detection of propionamide, C 2 H 5 CONH 2 , the next member in the homologous series, C n H 2n+1 CONH 2 , suggests that peptide-like molecules might be widespread in space; 7 however, Kolesniková et al. have not confirmed this claim and could detect neither propionamide nor the unsaturated prop-2-enamide or acrylamide, H 2 C --CH -CONH 2 , towards Sgr B2(N). Since nitriles are abundant in the ISM, Alonso et al. have speculated, that the hydrolysis of cyanoacetylene H -C ---C -C ---N, would lead to the formation of 2-propynamide, H -C ---C -CONH 2 ; however, the search was unsuccesful. Apart from N-methyl formamide no other species with molecular formula C 2 H 5 NO has been found and it is therefore of interest to document those isomers and provide some key background detail of these neutral species which contain in C, H, N and O -the four basic exobiological elements.