Two simple and sensitive spectrophotometric methods are described for the determination of benzalkonium chloride which is added as a preservative in certain dosage forms. The methods are based on the formation of highly coloured stable radical anions between benzalkonium chloride as n-electron donor and 7,7,8,8-tetracyanoquinodimethane (TCNQ) or 2,3-dichloro-5,6dicyano-p-benzoquinone (DDQ) as n-electron acceptors. Beer's Law is obeyed over the concentration range 0.5 - 8 and 1 - 8 μg ml−1for TCNQ and DDQ respectively. The optimum experimental parameters for colour production with both reagents were studied and incorporated into procedures. The percentage recoveries obtained were in accordance with those given by the official method. The proposed methods are characterized by high sensitivity, satisfactory accuracy and precision.