The first Li insertion into various gamma-MnO2 compounds, characterized by their rates of De Wolff and microtwinning defects, and the reversible Li intercalation process into the resulting gamma-LixMnO2 materials have been investigated. The first Li insertion into gamma-MnO2 is a first order reaction which occurs via a nucleation and growth mechanism. When the concentration of De Wolff defects increases, the thermodynamic potential of this reaction decreases and its kinetics becomes slower. The resulting gamma-LixMnO2 materials exhibit a very close structural relationship with so-called CDMO materials. The reversible Li intercalation process into gamma-LixMnO2 occurs in two steps that both operate via a two-phase mechanism. The concentration of De Wolff defects also plays a detrimental role in the kinetics of reversible lithium intercalation in electrochemically formed gamma-LixMnO2. Structural characterization of materials at the end of the first discharge have shown that gamma-MnO2 materials with a high rate of De Wolff defects are subjected to deeper structural rearrangements.