This review analyses some aspects of the variety of different structures small molecular compounds of the subvalent Group 13 elements can adopt. The discussion of experimental work restricts itself to matrix isolation studies. The experimental results are compared with those obtained for related (more stable) molecules using experimental methods other than matrix isolation and with the results of quantum chemical studies. Special consideration is given to the structural changes between homologues. All the molecules dealt with represent highly unstable species or reaction intermediates which can nevertheless be retained and characterized at leisure with the aid of the matrix-isolation technique. The combination of experimental spectroscopic information and detailed quantum chemical calculations can be used as a framework to analyse in detail the molecular and electronic structures of these species, most of which were sighted only very recently in experiments. Trends and differences in reactivity will also be considered, where information (experimental or theoretical) is available.