A single-line flow injection analysis (FIA) system for simultaneous determination of hydrolyzable metals and free acid has been developed. As a model for hydrolyzable metals iron was chosen. The setup allows determination of 0.5-6 M acid and 0.1-1.6 M iron. The resulting prediction error is 7% for acid and 3.8% for iron. Data were obtained by using a diode array detector. The calibration models were developed by using partial least-squares (PLS) regression. These results were compared with results obtained by traditional univariate calibration. The project aimed at developing a simple and robust FIA system to be applicable in an industrial process setting. Necessary modifications of present FIA instrumentation to fit such an environment are discussed.