.When compounds of Si , Ge and Sn are added to 2/C6H6 diffusion flames, ions characteristic of the elements are detected in the burnt gases by mass spectrometry. The ions, which are attributed to charge exchange and proton transfer reactions of H3O+, may be divided into three categories, atomic ions, protonated monoxides and protonated oxyacids . Sn gives atomic ions and protonated monoxides, a similar result to Pb , but neither Si nor Ge give atomic ions. This may be a consequence of the lack of Si and Ge atoms in the burnt gases. Ge and Sn give a protonated monoxide, but Si does not. This observation leads to the suggestion that proton affinities of the Group 4a monoxides are in the order CO, SiO < H2O < GeO , SnO , PbO. Protonated silicic and germanic acids are observed in the burnt gases of Si - and Ge -containing flames, but no oxyacids were observed in Sn -or Pb -containing flames. This indicates that the proton affinities of both acids are greater than that of H2O.