There is growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs). However, the possible reaction between the electrolyte and ammonia, and its potential effect on performance, has received little attention. In this paper, we report an investigation of the high temperature (950 °C) reaction of the apatite-type oxide ion conductor, La9.33Si6O26, and ammonia. The results show that such treatment leads to nitridation of the sample, with evidence for Si loss leading to an increased La:Si ratio in the final product. From neutron diffraction studies, the composition of the final product was determined to be La9.7(1)Si6O22.6(2)N2.7(2), with structural and 29Si NMR data suggesting the presence of N both within the apatite anion channels, and bonded to Si. An interesting feature of the structural studies are the relatively low atomic displacement parameters compared to the comparable apatite oxide systems, La9.33 + xSi6O26 + 3x/2, which can be related to the lack of interstitial anions in the oxynitride. Further studies on samples heated in ammonia at lower temperatures (600, 800 °C) suggest lower N incorporation, particularly for the 600 °C treatment. Considering the correlation of ionic conductivity, and interstitial oxide ion content in apatite systems, the data suggests the potential use of apatite-type electrolytes in SOFCs utilising NH3 as the fuel should be limited to temperatures <800 °C.