Temperature dependent electrical resistivity, crystal structure and heat capacity measurements reveal a resistivity drop and electrical transport behavior change corresponding to a structural change near 400 K in Ca(3)Co(4)O(9). The lattice parameter c varies smoothly with increasing temperature while anomalies in a, b(1) and b(2) lattice parameters occur near 400 K. The Ca site in the Ca(2)CoO(3) block becomes distorted and a change in electrical transport behavior is found above 400 K. Resistivity and heat capacity measurements as a function of temperature under magnetic field combined with Co L-edge x-ray absorption spectra reveal only a weak spin contribution to this change. Reduced resistivity associated with the structural change enhances the thermoelectric properties at moderately high temperatures and points to the electrical transport behavior change as a mechanism for improved ZT in this thermoelectric oxide.