Manganese(II) (Mn(II)) complex salts of poly(ethylene-co-methacrylic acid) (EMAA) were produced from a precursor Mn(II) salt of EMAA and eight different kinds of organic amines, and their oxygen-gas (O-2) sorption behavior was investigated using visible absorption and infrared spectroscopies. It was found that the Mn(II) complex salts exhibit a selective O-2 sorption during ageing at room temperature in air and release sorbed O-2 molecules from ionic aggregates of the Mn(II) complex ions above 343 K in vacuo. From these results, the relationship between the O-2 sorption behaviour and the chemical structure of amines is discussed.