In the present paper, the composition of several spinel-type λ-MnO2 samples (which absorb lithium ions with a high selectivity in an aqueous solution) is analyzed and the correlation between the composition and the absorption capacity of lithium is investigated experimentally. Then samples of λ-MnO2 with different hydrogen contents are prepared, and each lithium absorption capacity is measured. The results indicate that the absorption capacity of lithium is proportional to the content of hydrogen ions with the ability to ion exchange. Furthermore, it is concluded that the reason for the selective absorption of lithium ions in λ-MnO2 is not based upon the so-called ion sieve effect emphasized conventionally, but is based upon a proper ion exchange reaction which occurs only between lithium and hydrogen ions.