A commercial poly(acrylic acid) (PAA, M-nu = 150 000) has been labelled with low levels (0.4 to 3.1 mol%) of either pyrene, naphthalene or both chromophores introduced randomly along the chain. The photophysical properties of the polymers have been investigated by steady-state and time-resolved fluorescence spectroscopy. The pH-behaviour of the polymers has been examined by non-radiative energy transfer (NRET) measurements and by the ratio (I-E/I-M) of excimer to monomer intensities. The pH-induced expansion of the pyrene-labelled PAA is accompanied by a sharp decrease in the ratio I-E/I-M, signalling the disruption of pyrene excimer forming sites. In the case of the doubly labelled PAA, the extent of NRET, which monitors the increase in the naphthalene to isolated pyrene separation distance, occurs over a wide pH range (3.0 to 4.6). In contrast, the decrease in I-E/I-M, which reports on the fraction of pyrene groups located in excimer forming sites, takes place sharply at pH 4.5 which is also the polymer pK(a) derived from potentiometric titrations.