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Abstract: Rhodococcus erythropolis strain NTU-1 was isolated from oil-contaminated aqueous sludge and can grow at relatively high concentration of n-hexadecane as the only carbon source. The higher concentration of n-hexadecane, the faster degradation rate by NTU-1. Approximately 20,000ppmv (2%) of n-hexadecane were degraded in a treatment with 100,000ppmv (10%) within 4days of incubation. The amount of H+ ions released corresponded well to the carbon-chain length of the n-alkanes (either n-tetradecane, n-hexadecane or n-octadecane). The correspondence was 0.634mmol H+ ions accumulated per mole of n-hexadecane biodegraded. Using this correspondence, n-alkanes consumption can be closely estimated by monitoring pH changes in the medium. This procedure presents an alternative to other complex procedures that use organic solvent extractions and gas chromatography analysis. The procedure following the H+ ions accumulation was validated in a fed-batch bioreactor operation. [Copyright &y& Elsevier]