The fluoroquinolone antibacterial norfloxacin (NF) is a moderate photosensitizer of singlet molecular oxygen (1O2). We have studied photosensitization by NF as a function of medium polarity and proticity in solvent mixtures. We have used 1,4-dioxane and propylene carbonate mixtures to keep proticity constant while modulating polarity, and water/D2O and ethylene carbonate mixtures to alter proticity without large changes in polarity. The absorption spectrum of NF was little affected by solvent changes, as compared to the fluorescence spectrum that exhibited as much as a 50 nm blue-shift, e.g. 1,4-dioxane versus D2O. The quantum yield of NF fluorescence saturated at an almost 10 times higher value (approximately 0.14) when proticity was increased by added water, up to 0.2 mol fraction, to ethylene carbonate. Less pronounced, the increasing polarity in 1,4-dioxane/propylene carbonate mixtures affected the fluorescence yield much less. Norfloxacin produces 1O2 and is able to quench 1O2. The rate constant for 1O2 quenching is 4.5 x 10(7) M-1 s-1 in propylene carbonate but decreases ca four times in D2O. The quantum yield of 1O2 photogeneration was also up to five times higher in solvents that were both protic and polar than vice versa. Our data show that NF is more photochemically active in an environment that is both protic and polar. This suggests the involvement of polar excited state(s) and possible proton/hydrogen transfer during photoexcitation. Similar processes may initiate the phototoxic response reported in some patients treated with the fluoroquinolone drugs. The phototoxicity of NF and other fluoroquinolone antibiotics may strongly depend on their localization in hydrophilic or hydrophobic cell/tissue regions.