Treatment of wastewater from a Pinus radiata sulfite pulp and paper mill with coagulation followed by aerated stabilization basins (ASBs) was investigated to determine a treatment process that minimizes the colour formation. Two post-coagulation wastewaters were investigated, one low in P (Simulation A; BOD:N:P = 100:1.3:0.06) and the other higher in P but still moderately P-limited (Simulation B, BOD:N:P = 100:1.3:0.3). Changes in the organics character of the wastewaters were investigated at several stages of treatment: untreated, after coagulation; during and after ASB treatment, using HPSEC and solid-state (13)C CP NMR spectroscopy. Effective reductions in colour(456nm),. and UV(254) (and 280nm), (similar to 80%) were achieved by coagulation using alum (1860 ppm). However, during ASB treatment, colour(456nm) increased (>100%) in both simulations. HPSEC analysis showed that removal of HMW (>3000 Da) was achieved through coagulation with the simultaneous removal of LMW (<300 Da) in the ASB simulations. Subsequent to the removal of HMW and LMW, an increase of intermediate-range compounds (300-3000 Da) occurred. Solid-state (13)C NMR analysis indicated some selective removal of aromatic and alkyl groups during the coagulation process and additional removal of aromatic C during ASB treatment. The slightly higher proportion of HMW (>1000 Da) in 'A' than 'B' and the slightly higher proportion of aromatic C and alkyl groups remaining in the effluent of 'A' might be the basis for differences in the colour development in the two ASB simulators. The low nutrient loading (external) in the pre-treated wastewaters has been shown to be sufficient to sustain microbiological growth of organisms that contribute to colour formation with ASB treatment. This occurred in both simulations and it was concluded that the manipulation of the BOD:N:P ratio to optimise BOD removal had little effect on colour removal. This paper provides insight on the nature of recalcitrant organic compounds causing colour formation. (C) 2011 Elsevier B.V. All rights reserved.