Two benzimidazole anthelmintic drugs; flubendazole (FBZ) and mebendazole (MBZ), were determined by simple spectrofluorimetric method through complex formation with erythrosine B. The ion-pair formation between the dye and the studied drugs quench the fluorescence activity of the dye which was monitored at 554 nm (ex =527 nm). The reaction was performed in Teorell - Stenhagen buffer solution (pH 2.9). The reduction in the emission value of the fluorescence was proportional to the added drug in a linear range of 0.1–3.5 µg mL−1 for both drugs. The detection limits were 34 and 30 ng mL−1 for FBZ and MBZ respectively. The suggested procedure was validated in accordance with ICH guidelines, with satisfactory results. Several commercially available pharmaceutical formulations containing the drugs were successfully analyzed with no significant interference due the commonly used excipients. In addition, the quenching mechanism was explored with Stern Volmer equation and the quenching of the fluorescence was assumed to proceed through static process. Moreover, the free energy change, ΔG°, for the complex formation reactions were −25.6 and −30.1 KJ mol−1 for FBZ and MBZ, respectively.