Continuous co‐biodegradation of a linear and three cyclic surrogate naphthenic acids namely octanoic acid, trans‐4‐methyl‐1‐cyclohexane carboxylic acid (trans‐4MCHCA), and cis‐ and trans‐4‐methyl‐1‐cyclohexane‐acetic acids (cis‐4MCHAA and trans‐4MCHAA) at concentrations of 100, 50, and 50 mg/L, respectively, was investigated in two circulating packed‐bed bioreactors. Using different combinations of naphthenic acids in the bioreactor influent effects of loading rate on biodegradation of each compound was evaluated. In all tested mixtures biodegradation of octanoic acid proceeded with the fastest rate, followed by trans‐4MCHCA, trans‐4MCHAA, and cis‐4MCHAA. Biodegradation of the linear compound (octanoic acid) was not influenced by the presence of any cyclic compound, even the most recalcitrant naphthenic acid with the highest removal rate of 372.8 mg/L‐h observed at a loading rate of 666.7 mg/L‐h. Presence of octanoic acid negatively impacted the biodegradation of trans‐4MCHCA, with the removal rates of trans‐4MCHCA in the presence and absence of octanoic acid being 74.6 and 208.8 mg/L‐h, respectively. With cis‐ and tran‐4MCHAA, presence of octanoic acid improved the biodegradation of these compounds. In a mixture containing all three cyclic compounds, the maximum removal rates for cis‐ and trans‐4MCHAA were 0.5 and 1.3 mg/L‐h, respectively, which improved to 1.7 and 2.4 mg/L‐h when octanoic acid was included in the mixture. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 835–843, 2014