The speciation of Al-OH complexes in terms of Al(a), Al(b) and Al(c) could be achieved by traditional ferron assay and Al(b) is generally considered as Al(13), however, the inherent correlation between them remains an enigma. This paper presents a modified ferron assay to get precise determination of Al(13) using nonlinear least squares analysis, and to clarify the correlation between Al(b) and Al(13). Two parallel reactions conforming to pseudo-first-order kinetics can simulate the complicate reactions between polynuclear complexes and ferron successfully. Four types of experimental kinetic constant (k value) of Al-OH complexes can be observed by this method when investigating three typical aluminium solutions. Comparing with the results of (27)Al NMR, the species with moderate kinetics around 0.001 s(-1) can be confirmed to resemble to Al(13) polycation. The other types of kinetics are also well-regulated in partially neutralized aluminium solutions with various OH/Al ratios (b values) in the range 0 approximately 2.5. It would provide potential means to trace the in-situ formation of Al(13) in dilute solutions such as coagulation with Al-based coagulants.