This study describes the preparation of sodium morin–5–sulfonate (NaMSA) as a new reagent for the determination of aluminium(III) (Al3+) ions based on the formation of a ternary complex. The complex consists of Al3+, NaMSA, and 1–Butyl– 3–methylimidazolium hexafluorophosphate (BMIM–PF6). It was found that this method was sensitive compared to the binary complex of Al3+ and NaMSA. The ternary complex was excited at 420 nm, and the fluorescence signal was measured at 518 nm. Maximum fluorescence signal produced at pH 5.0 (acetic acid–acetate buffer), with 0.02% v/v BMIM–PF6 and 1.0 × 10-4 molL-1 of NaMSA. The calibration graph in linear up to 10 mgL-1 with the calculated detection limit of 0.017 mgL-1. Effect of foreign ions towards the ternary complex was also studied. Finally, the method was applied in the determination of Al3+ ions in water samples, and satisfactory results were obtained.