Abstract Thorium is an important element not only from an environmental perspective but also in industrial applications. In this study, Arsenazo III, Thorin and Thionin were used as complexation reagents for the determination of Thorium using flow cell, dip probe and microchip systems [P. Caglar, S.A. Tuncel, N. Malcik, J.P. Landers, J. Ferrance, A microchip sensor for calcium determination, Anal. Bioanal. Chem. 386 (2006) 1303–1312]. For this aim, the spectral characteristics of Th 4+–dye complexes, and the effects of pH on the formation of Th 4+–Ars III, Th 4+–Thorin and Th 4+–Thionin complexes have been determined. The determination of the complex stoichiometries and the complex formation constants were also studied using the three systems. The complex stoichiometries were found to be 1:1 for all ligands and for all systems under study in this paper. Nevertheless, the formation constants ( K f) have been determined as 2.340 × 10 6, 8.156 × 10 5, and 6.732 × 10 5 mol L −1 for Th 4+–Ars III, Th 4+–Thorin and Th 4+–Thionin complexes, respectively, using the dip probe system. The K f values were found to be 9.422 × 10 4, 6.752 × 10 4, and 4.199 × 10 4 mol L −1 for Th 4+–Ars III, Th 4+–Thorin and Th 4+–Thionin complexes, respectively, for the flow cell system, whereas these values were determined as 8.526 × 10 4, 5.988 × 10 4, and 3.892 × 10 4 mol L −1 for Th 4+–Ars III, Th 4+–Thorin and Th 4+–Thionin complexes, respectively, using the microchip system. The response times of the sensors, the linear range of the sensor response, the detection limits and the relative standard deviation (RSD) of the sensors have also been found.