New ion-conducting polymer composite films have been prepared, and their ionic conducting properties have been investigated. The polymer composite films are fabricated from partially phosphorylated poly(vinyl alcohol) with tetramethylammonium salt (P-PVA(.)Me(4)N(+)) and poly(acrylic acid) (PAA) or poly(ethylene glycol) (PEG). For P-PVA(.)Me(4)N(+)/PEG composite films, the ionic conductivity and carrier density sharply increased, and carrier mobility sharply decreased around [PEG]/[PO3](P-PVA) of 2. The ionic conductivity is dominated by both carrier density and carrier mobility at [PEG]/[PO3](P-PVA) < 2 and only by carrier density at [PEG]/[PO3](P-PVA) > 2. This is attributed to the fact that the ionic conduction in P-PVA(.)Me(4)N(+)/PEG composite films occurred through the PEG-Me4N+ complex which was independent of the carrier mobility. On the other hand, the ionic conductivity in P-PVA(.)Me(4)N(+)/PAA composite films showed a bell-shaped dependence on the PAA contents with a maximum value at [CO2H]PAA/[OH](P-PVA) = 1. FTIR spectrum measurements demonstrated that part of the carboxylic acid residues was dissociated in the composite films. This fact implied that the ionic conduction was mediated by PAA at the low PAA content. At high PAA content, however, an excess of the carboxylic acid residues formed trapping sites for the Me4N+ ion, leading to a decrease in the ionic conductivity. Furthermore, we proposed a unique mechanism of the ionic conduction.