A very simple, automatic, fast method for the photometric determination of bromhexine hydrochloride based on the application of kinetic methodology and the stopped-flow mixing technique to the coupling reaction between the diazotized bromhexine derivative and N-(1-naphthyl)ethylenediamine is proposed. The high initial rate of this reaction allows analytical measurements to be made within only 0.5–1 s, which makes the method applicable to automatic routine analyses. The calibration graph is linear over the range 1.5–65.0 μg mL−1and the precision (as %RSD) is less than 2%. The presence of antibiotics such as penicillins and cephalosporins in a 100-fold excess has no effect on the analyte determination. The proposed method was satisfactorily used for direct analysis of pharmaceutical formulations containing these antibiotics.