Neutron diffraction data, measured for two thallium germanate glass compositions, are presented and compared with previously published data for caesium germanate glasses. The measured coordination number, n(Ge-O), for the 10 mol%Tl2O thallium germanate glass is in good agreement with the average coordination number measured for the equivalent caesium germanate. However, while n(Ge-O) declines for caesium germanate glasses as the amount of modifier increases above 18 mol% Cs2O, n(Ge-O) for the 30 mol% Tl2O thallium germanate glass remains high with, on average, 4.40 +/- 0.03 oxygen neighbours per germanium. The difference in behaviour of the germanium coordination, compared to caesium germanate glasses, arises from a change in the average thallium environment, as the role of the thallium changes from that of a modifier (similar to an alkali), to a role where some of the thallium atoms act as network formers. This is supported by Tl-205 NMR measurements which indicate the presence of two thallium environments. (C) 2010 Elsevier B.V. All rights reserved.