A new method for measuring exciton diffusion length in nanoparticles (NPs) of conjugated materials is presented. Cationic acceptor dyes are used to quench the photoluminescence in NPs of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Amplified quenching of MEH-PPV emission is observed with an initial Stern-Volmer constant in excess of 10(5) M-1. Stern-Volmer plots are nonlinear with two distinct quenching regimes, hinting saturation of NP surfaces with acceptor molecules at some point during titration experiments. Using an assumption that highly efficient quenching of excitons occurs after saturation with acceptors at the NP surfaces, the amount of maximum emission quenching can be compared with a model of exciton diffusion to determine exciton diffusion length. By assuming quenching efficiency above 80%, the measured three dimensional exciton diffusion length is estimated to be 12 +/- 1 nm. This result is in the lower region of reported values ranging from 10 to 25 nm in MEH-PPV thin films. Both the derived model and the experimental methodology allow robust measurement of exciton diffusion length for any luminescent conjugated material from which NPs can be prepared. (C) 2013 AIP Publishing LLC.