The instrumentation development of a spectrophotometer based on an acousto-optic tunable filter (AOTF) and its applications in a flow-injection (FI) system for gradient exploitation is described. This compact, all-solid state spectrophotometer which contains no-moving parts, can be scanned in the visible region (390-650 nm) at a speed of 300 nm/s. The fast scanning capability of the instrument made it possible to monitor the spectral response of an acid-base indicator mixture. The recorded spectra of the indicators were then converted to pH by means of a partial least square (PLS) multivariate calibration technique. With use of the same indicator mixture, the relationship obtained facilitates the determination of pH of any now-injection pH gradient system which is produced by injecting a strong acid or base over a mixture of buffered solutions with a wide range of pH values. The time versus pH relationship can, therefore, be readily established for any buffer-reagent mixture flowing in the Fl system. The FI system, which has been calibrated this way, can then be used to rapidly characterize the behavior of chemical systems as a function of pH. Two examples are demonstrated in this work: the determination of the pK(a) of the weak acids and of the complex formation between iron(III) and sulfosalicylic acid.3193315324