Catalytic wet air oxidation of two acidic and alkaline Kraft bleach plant effluents (total organic carbon (TOC) content 1138 and 1331 mg l(-1),respectively) was investigated in a trickle-bed reactor at T = 463 K and oxygen partial pressure of 8 bar. The reactor packed with titania or titania-supported ruthenium catalyst was operated in a low-interaction (LIR) trickle-how regime either in continuous-flow or batch-recycle mode. In the off-gas, no carbon monoxide was detected in any of the runs. When the catalytic bed was composed of TiO2 particles, moderate abatement of organic compounds from the bleach plant effluents was observed (TOC conversions up to 46 and 26%, respectively). The removal of parent organic material was further enhanced by deposition of metallic ruthenium (3 wt.%) on the titanium oxide support. In that case, the once-through oxidation produced decolorized outlet streams with TOC conversions as high as 89 and 88%, respectively These values further increased to 98 and 95%, respectively by running the trickle-bed reactor in the batch-recycle mode of operation; the residual carbon content in treated effluents was found in the form of acetic acid. No leaching of Ru or Ti was detected by ICP-AES analysis to the detection limits of 0.2 and 0.1 mg l(-1), respectively.