The structures of two fast-ion-conducting molecular glasses, and , have been investigated by neutron and x-ray diffraction. Both molybdate and tungstate glasses show a first sharp diffraction peak (FSDP) at very low Q-values ( and respectively) in the neutron data. However, only weak peaks are observed at similar Q-values in the x-ray data. The diffraction results have been used to model the structures using the reverse Monte Carlo (RMC) method. The RMC results show that all partial structure factors contribute to the FSDP except , , and (or ). This implies that the origin of the FSDP is somewhat different from that for the borate and phosphate glasses where and (the corresponding centres of the covalently bonded oxide groups) contribute strongly to the prepeak. The result excludes any simple explanation of the FSDP in terms of AgI clustering.