Steam reforming (SR) of palm oil mill effluent (POME) over net-basic LaCoO3 was optimised for syngas production (F-syngas) and degradation efficacies (X-P) by tuning temperature (T), POME flow rate ((V) over dot(POME)), catalyst weight (W-c(at)), and particle size (d(cat)). Net-basicity of LaCoO3 facilitated the adsorption of Lewis acid CO2, thereby assisted carbon removal via reverse Boudouard reaction. POME SR over LaCoO3 was promoted by using (i) higher T (endothermicity), (ii) greater (V) over dot(POME) (larger partial pressure at constant weight-hourly-space-velocity and total feed rate), (iii) larger W-cat (longer residence time for POME vapour), and (iv) smaller d(cat) (higher surface area to volume ratio). Nevertheless, the catalytic activity of LaCoO3 declined with (i) severe coking and sintering deactivation (T >= 973 K), (ii) carbon-encapsulation ((V) over dot(POME) = 0.10 mL/min), (iii) agglomeration (W-cat>0.3 g), and (iv) pore occlusion (d(cat) <74 mu m). Hence, the optimum conditions of POME SR over LaCoO3 were T = 873 K, (V) over dot(POME) = 0.09 mL/min, W-cat = 0.3 g, and d(cat) = 74-105 mu m. The optimised process able to produce syngas at a rate of 86.60 mu mol/min whilst degrading POME to a less polluted liquid condensate (COD = 435 mg/L and BOD5 = 62 mg/L). (C) 2019 Elsevier Ltd. All rights reserved.