Al-13 is notable for its promising application in many fields. It has been believed that Al-13 could be assayed in term of Al-b derived from traditional ferron assay. But the inherent relation between Al-13 and Al-b is not clear. Here, the new k-value-based ferron assay using nonlinear least squares analysis is suggested to resolve the ambiguity. The experimental results reveal that the moderate kinetics (k value) around 0.001 s(-1) can be ascribed to Al-13. In the short-term aging of freshly neutralized aluminum solutions with OH/Al molar ratio of 2.2, the rapid progress of Al-13-like transformation into Al-13 can be traced by this method, whereas it may be masked by traditional ferron assay because the metastable intermediates also contribute absorbance to Al-b fraction. Al-b can only be regarded as Al-13 when Al-13 forms completely and becomes the stable specie in the matured solution. (C) 2009 Elsevier Inc. All rights reserved.