The crystal structures of triphenylgallium and triphenylindium have been determined. Both compounds crystallise with Z= 4 in a unit cell of space group Pbcn, but they are not iso-structural. In the indium compound short intermolecular contacts suggest that weak association occurs by donation of electrons from a phenyl group of one molecule to the vacant p-orbital of indium in another molecule. Similar but weaker association may also occur in the gallium compound.