Photon pair-correlation spectroscopy was used to study the photoluminescence of single, isolated chains of the conjugated polymer MEH-PPV (poly[2-methoxy,5-(2-ethyl-hexyloxy)-p-phenylene-vinylene]). The chain conformation of these multichromophoric molecules was controlled by varying solvent polarity to result in either tightly folded collapsed chain or extended chain structures. We demonstrate, that the polymer conformation has strong influence on the quantum optical nature of the single chain photon emission. Polymers in the collapsed-chain conformation exhibit the non-classical phenomenon of photon antibunching in contrast to extended-chain structures. Analysis of the second-order correlation function provides a quantitative measure of the number of active emitter sites. (C) 2003 Elsevier Science B.V. All rights reserved.