In this contribution, we report a versatile method for tuning optical properties of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in its solution with 1,2-dichloroethane, accomplished by reacting with pyridinium formate (PF), a volatile organic salt. We can systematically control the positions of absorption and photoluminescent (PL) spectra of MEH-PPV by adjusting the concentration of PF in the solution. The addition of 10 vol % PF caused a blue-shift in the absorption spectra by about 65 n-m. When the concentration of PF decreased to 0.1 vol %, the blue-shift occurred to a lesser extent, about 25 nm. The measurements of PL spectra showed similar behaviors. The lambda(max) shifted from 558 nm to 546 and 552 nm when 10 and 0.1 vol % of PF were added, respectively. The changes of PL colors from orange to yellow and green, respectively, were observed by naked eyes. Structural investigation by nuclear magnetic resonance and Fourier-transformed infrared spectroscopy indicated that the changes of the optical properties were due to chemical modifications along the main chain and the side groups of MEH-PPV. These results implied a simple route for engineering the HOMO-LUMO energy gap of MEH-PPV, which could be utilized in advanced applications such as organic light-emitting devices and solar cells. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polyrn Phys 47: 696-705, 2009