An investigation of the mechanism of the fluorescence quenching by Cu(2+) for a conjugated polymer system initially designed as a fluorescence "turn-on" chemosensor based on chelation enhanced fluorescence (CHEF) is described in this paper. Unlike all other metal cations tested, the polymer/Cu(2+) hybrid system with a 1:1 ratio between the receptor and Cu(2+) has only weak fluorescence with lambda(max) = 490 nm and a quantum yield of 0.004 in THF at room temperature. In solvent glasses at 77 K the fluorescence remained quenched suggesting that the quenching mechanism was due to energy transfer between the Cu(2+) and the conjugated polymer backbone. The energy transfer quenching competes effectively with the electron transfer involved in the CHEF resulting in a more selective chemosensory system.