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The dynamic method for monitoring ionisation of the lignin functional groups in a 0.1 N NaOH solution at ambient temperature by means of the SEC technique with multiwave UV detection was performed. An increase in absorbance at 210, 278, 304, 320 and 350 nm was examined to reveal the ionisation dynamics in respect to functional groups’ acidity; that is, carboxyl groups, carbonyl- and double-bond conjugated phenol hydroxyls. A distinctive response of absorbance at 210 and 278 nm on the ionisation of carbonyl- and double-bond-conjugated phenols was found. Simultaneous characterisation of lignin by molecular weight (MW) and chemical heterogeneity was performed. A non-uniform distribution of reactive sites over the MW distribution for commercial kraft lignin (Curan) was found employing the family of chemical composition distribution curves. The redistribution of extended chromophore structures over time along the lignin’s MW distribution causes a misrepresentation of the UV chromatogram pattern and influences the accuracy of MW values.