Apatite-type oxides of general formula (La/Sr)(10-x)Si6O26+y have been attracting considerable interest recently, because of their observed high oxide ion conductivity. In this paper, we report the effects on the conductivity of Ga doping at the Si site. For samples that are nominally stoichiometric in oxygen, La9.33+x/3Si6-xGaxO6 (0 less than or equal to x less than or equal to 2), we find that the oxide ion conductivity increases with Ga content up to a maximum value (sigma(500 degreesC)= 1.3 x 10(-3) S cm(-1)) for x = 1.5, before decreasing with further Ga incorporation. The conductivity data is discussed in relation to results from other doping studies and neutron powder diffraction structural data collected for the samples La9.67Si5GaO26 and La10Si4Ga2O26. In terms of the latter, the data shows a difference in space group (from P6(3) to P6(3)/m) between the samples. In addition, there is a larger anisotropy of the thermal displacement parameters for the channel oxygens in La9.67Si5GaO26, which is consistent with the higher conductivity of this sample. (C) 2004 Elsevier B.V All rights reserved.