Salt-containing poly(ethylene oxide) (PEO) was used as a solvent for electrochemical redox reaction of poly(pyrrole) (PPy). The potential window of PEOs containing LiClO4 was confirmed to reach a constant range from - 1.60 to + 1.60 V (versus Ag) when their ionic conductivity exceeded 3.0 x 10(-4) S/cm. The p-doping behavior of PPy in PEOs was further influenced by the ionic conductivity even at higher than 3.0 x 10(-4) S/Cm. In other words, it depended on the interaction between PPy and salt. Then the effect of ion species on the p-doping behavior of PPy was analyzed in PEO. Perchlorate anion (ClO4-) gave the highest peak current reflecting the largest doping in PPy. The effect of PPy film thickness on the p-doping varied with anion size reflecting a steric hindrance of larger anion and an interaction force between anions and cationic sites on PPy. Taking these opposite factors into account, perchlorate anion (ClO4-) was concluded to be the best dopant anion for p-doping of PPy in PEO oligomers.