In order to know the effect of simple anions on the formation of a poly(tetraisopentylammonium acrylate) ((i-C5H11)(4)NPA) hydrate, a thermal analytic method using a differential scanning calorimeter was applied to (a) aqueous solutions of (i-C5H11)(4)NPA whose PA anion is partially replaced with simple anions like Cl- and C2H5COO-, (i-C5H11)(4)N[PA]Cl-1-x(x) and (i-C5H11)(4)N[PA](1-x)[C2H5COO](x) (x = 0-1.0), and to (b) aqueous solutions of (i-C5H11)(4)NPA containing either NaCl or C2H5COONa. II was found that when Cl- ion coexisted with the PA anion, two kinds of hydrates, i.e. (i-C5H11)(4)NPA hydrate and (i-C5H11)(4)NCl hydrate, were formed separately, and a solid solution phase containing both (i-C5H11)(4)NPA hydrate and (i-C5H11)(4)NCl hydrate was not detected. In addition, in the (i-C5H11)(4)NPA + NaCl systems, the amount of (i-C5H11)(4)NCl hydrate increased and the PA anion was gradually expelled from the hydrate phase on increasing the amount of NaCl. The behavior of the C2H5COO- ion was almost the same as that of the Cl- ion.