The photophysics of a number salicylic acid derivatives (SADs) in aqueous solutions was investigated in a wide range of pH by time-correlated single photon counting (ex = 350 nm, resp = 300 ps) and fluorescence up-conversion (ex = 266 nm, resp = 300 fs) techniques. The acid-base equilibrium constants in the ground (pKa) and the excited states ([]), the fluorescence quantum yields as well as the lifetimes of anionic, neutral, and cationic forms of SADs were determined. Evidence of ultrafast excited-state intramolecular proton transfer (ESIPT) leading to the formation of the proton-transferred excited state of SADs was obtained from the fluorescence up-conversion measurement. The nature of the ESIPT process is discussed.