Extraction of M(ClO4)m, (Mm+= Li+, Na+, Mg2+, and Ca2+) from their aqueous solutions into hexane with trioctytphosphine oxide (TOPO) and the co-extraction of water have been measured at 298 K. The solvation number of TOPO is concluded to be 3 for Li(l) and Na(I), 4 and 7 for Mg(II), and 4 and 6 for Ca(II) from the curve-fining calculations.. The hydration number of the extracted metal salts is 2 - 3 for Li(l), Na(I), and Ca(I) but 6 - 8 for Mg(II). The hydration number seems to increase on increasing in the TOPO concentration. It is deduced that TOPO solvates both the metal ion and waters of hydration of the metal ion.