In this work, the lowest excited singlet states of acridine (Acr), acridinium (AcrH+) and 10-methylacridinium (AcrMe+) are quenched by sulfur-containing amino acids and carboxylic acids in aqueous solution. Both steady-state and time-resolved fluorescence techniques were used to monitor the quenching of fluorescence. Stern–Volmer plots of the fluorescence intensity showed a static component (KS) to the quenching. The experimental KS values were compared to theoretical KS values for outer-sphere complexes based on Debye–Hückel theory and the Fuoss equation. The general agreement between experimental and theoretical KS values indicate that the static quenching can be attributed to non-fluorescing ion pairs associated as simple outer-sphere complexes. The computed values of the interionic distances of the ion pairs are consistent with the ion pairs of the ZAZQ=−1 and −2 cases being solvent-separated ion pairs while those of the ZAZQ=−3 case are contact ion pairs. The effect of the reactants’ charges on the quenching rate constants (dynamic component) was observed for the reactions of AcrMe+ with the anionic forms of the quenchers (having charges ZQ=−1, −2 and −3). The rate constants (extrapolated to ionic strength, μ=0) for the quenching processes were determined to be 0.3–5.3×1010 M−1 s−1 depending on the ionic charge (ZQ) of the quencher used. These trends in the quenching rate constants are rationalized with a quenching scheme for electron transfer. Analogous quenching rate constants for alanine and glycine were found to be at least an order of magnitude lower. Photoinduced electron transfer from the sulfur atom of the quencher molecule to the acridine excited singlet state is suggested to be the most likely mechanism of the process under discussion.