The effects of lithium ion insertion on the structural and electronic properties of FeS2 (pyrite) have been examined in situ by Fe K-edge X-ray absorption fine structure (XAFS) using electrodes and electrolytes similar to those found in conventional, ambient-temperature, primary Li/FeS2 batteries. A substantial reduction in the amplitudes of the Fe-S and Fe-Fe backscattering was observed as the amount of intercalated lithium in the FeS2 lattice was increased from 0 to 2 Li+ equivalents, (Li+)(eq). After the insertion of 2 (Li+)(eq) the second-shell Fe-S interaction and the distant Fe-Fe interactions were no longer discernable in the Fourier transform (FT) data, Curve-fitting analysis of the k(chi)(3)(k) extended X-ray absorption fine structure for this latter material yielded an average Fe-S distance, d(Fe-S) = 2.31 +/- 0.02 Angstrom A, which is about 0.05 Angstrom A longer than d(Fe-S) in crystalline pyrite. In addition, the X-ray absorption near-edge structure revealed a rounding of the otherwise highly structured edge of FeS2 as the amount of inserted lithium was increased, This behavior is consistent with the formation of Fe1-xS and thus supports the assignment made on the basis of in situ Fe-57 Mossbauer effect spectroscopy of the same system.