Pyrazolo[3,4-b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light-emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H2O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady-state and transient fluorescence spectra of 1-phenyl-3,4-dimethyl-1H-pyrazolo-[3,4-b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground-state proton-transfer complex [PAQ5⋅⋅⋅H+] is responsible for static quenching. It changes linearly with proton concentration [H+] with a bimolecular association constant KS=1.95 M−1 controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant KD=22.4 M−1 is obtained by fitting to the Stern–Volmer equation, with a bimolecular dynamic quenching rate constant kd=1.03×109 s−1 M−1 under ambient conditions. A change in electron distribution is simulated and explains the experiment results.