Regioregular poly[(p-phenylene ethynylene)-alt-(2,5-thienylene ethynylene)]s (PPETEs) have been synthesized to evaluate the pure regioregularity effect on the physical properties of pi-conjugated polymers. Comparison of their UV-vis and C-13 NMR spectroscopy confirms the absence of steric interaction between the adjacent substituents on thiophene rings. Although both polymers emit at the same wavelength, PPETE 3 with a pure head-to-tail chain sequence exhibits a slightly stronger fluorescence from its solution than PPETE 4 with a head-to-head chain sequence (phi(fl) = 0.59 for 3 vs 0.53 for 4). The Mark-Houwink constant alpha and radius of gyration R-g values have been determined for PPETEs 3 and 4, both indicating that 3 has a more rigid conformation than 4. The higher molecular rigidity for 3 might be partially responsible for its higher PL quantum efficiency. A significant enhancement in PL efficiency has been observed from PPETE films. Under identical experimental conditions, a film of 4 emits about 2.5 times stronger than 3 and 6 times stronger than the regiorandom PPETE, respectively.