Abstract The fluorescence properties of 2-acetylnaphthalene (2-AN), 1-acetylnaphthalene (1-AN) and 5-(4-bromo-1-naphthoyl)pentyltrimethylammonium bromide (BNK5 +) in water have been investigated. The origins for this study are conflicting reports in the literature concerning the fluorescence behavior of the latter two molecules. We have determined that slight contamination (∼1%) of these molecules with the corresponding 2-isomer can result in a fluorescence emission dominated by the impurity. When 1-AN and BNK5 + are extensively purified, they both show fluorescence maxima in the region 425–430 nm, only slightly blue-shifted ( <8 nm) from the maximum for 2-AN fluorescence. However, the fluorescence efficiencies of 1-AN and BNK5 + are less than 1% of that for 2-AN. These results suggest a similar charge-transfer dominated emitting state for all three molecules, while the energies of the (n,π ∗) states responsible for intersystem crossing may be isomer-dependent.