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Thermodynamic and kinetic aspects of the formation of trihalomethanes and haloacetic acids determined based on the quantum chemical (QC) simulations were compared in this study with the experimental data generated using the differential spectroscopy approach in chlorination and chloramination. The ratios of the slopes of the correlations between -DlnA 350 values and individual DBPs concentrations (S NH2Cl /S HOCl) were observed to be linearly correlated with the ratios of the Gibbs free energies (ΔG NH2Cl /ΔG HOCl) of the corresponding reactions of chloramine and chlorine with acetaldehyde which was used as a model DBP precursor in QC simulations. Further QC examination of the kinetics of chlorination and chloramination of the model compound acetoacetic acid showed that the activation energy of reactions between monochloramine that directly participates in substitution reactions to form mono-, di and tri-halogenated intermediates are 2–3 times higher than those of HOCl formed via the hydrolysis monochloramine. This result confirms that the interactions of chloramine with NOM and ensuing DBP formation are primarily mediated by the free chlorine released as a result of the hydrolysis of monochloramine while direct halogenation of NOM by monochloramine is likely to provide a small contribution to DBP formation. Image 1 • -DlnA 350 is linear with most of individual DBPs yields in chlor(am)ination. • Ratio of slopes (S NH2Cl /S HOCl) for individual DBPs are linear with ΔG NH2Cl /ΔG HOCl. • Direct halogenation of NOM by NH 2 Cl contribute to small fraction of DBP formation. • The activation energy of NOM reaction with NH 2 Cl is higher than HOCl. • Tri-chlorinated HAAs and THMs are suppressed in chloramine due to trace level HOCl. [ABSTRACT FROM AUTHOR]