Lithiated FeS2 samples with various lithium contents were examined by a combination of ex situ X-ray powder diffraction and ex situ scanning electron microscopy to provide experimental evidence for intermediate phases and insights to lithium reaction mechanisms of pyrite at ambient temperature. Natural and synthetic FeS2 samples having distinctly different grain sizes were used in this study. Structural and morphological changes of these FeS2 samples were compared under low and high current densities. Structural and morphological evidence revealed the presence of intermediate phases upon two-voltage-step lithium reduction of FeS2 pyrite at ambient temperature. These intermediate phases were similar to some of those obtained from lithium reduction of FeS2 near 400degreesC. Of significance was X-ray powder diffraction evidence of a layered Li2+xFe1-xS2 (0 less than or equal to x less than or equal to 0.33) solid solution phase upon the 1.7 V lithium reduction of FeS2 at ambient temperature. A lithium reaction mechanism for two-voltage-step lithium reduction of FeS2 pyrite was proposed and compared to those proposed in the literature. In addition, it was found that one-voltage-step lithium reaction of FeS2 pyrite at 1.5 V or lower led to the formation of plate-like Li2S (plate thickness on the order of 20 nm) and Fe, which supported the reaction mechanism proposed by Fong et al. [J. Electrochem. Soc., 136, 3206 (1989)].