Fluorescence response of a photoacid inside a confined environment often differs markedly from the bulk response. Is there any correlation between the extent of fluorescence modulation and the strength of the photoacid? Here, we used three photoacids: 2 -naphthol (2OH, pK(a)* = 3.3), 6-sulfonate-2-naphthol (6SO(3) -20H, pK(a)* = 3.06), and 6-cyano-2naphthol (6CN-2OH, pK(a)* = 0.6) with remarkably different excited-state acidities to investigate fluorescence modulation inside the nanocavity of beta-cyclodextrin (beta-CD). Interestingly, we found strong fluorescence modulation for 20H and 6SO(3)-20H but almost none for 6CN-20H. Isothermal calorimetry measurements showed that all three fluorophores form 1:1 inclusion complex with comparable binding constants (285, 420, and 580 M-1 for 20H, 6SO(3)-20H, and 6CN-2OH, respectively). Molecular dynamics simulation further revealed that binding modes are quite similar, and the distribution of water molecules around the proton -donating hydroxyl group of the photoacids are also comparable. Consequently, the difference in the fluorescence response should be accounted solely to the difference in the photoacidity strengths.