We have developed synchrotron X-ray diffraction techniques for studying the structure of silicate melts and have applied them to measuring the structures of MgSiO3 and CaSiO3 melts up to 6 GPa. In both melts, it appears that the changes in the local structure around the divalent ions and in the network structure of SiO4 tetrahedra are the dominant compression mechanisms while a change in the local structure around the Si ion is a secondary mechanism up to at least 6 GPa. The structural changes appear to be larger in CaSiO3 melt than in MgSiO3 melt, especially in the vicinity of ambient pressure. These results are consistent with available experimental and theoretical information on MgSiO3 and CaSiO3, and explain discrepancies in previous work.