The method of decomposition of chlorinated organic substances in contaminated water based upon successive steps of chemical pretreatment of organic compound with a) active radicals ( Fenton reaction) and/or with b) reductive dehalogenation on metallic Pd in the presence of zero-valent iron, followed by aerobic biodegradation using bacteria strain Pandoraea sp. was studied. 4-chlorophenol was chosen as a model compound. Generally, chlorophenols show limited biodegradability. The average efficiency of biodegradability of 4-chlorophenol with both free and immobilized cells does not exceed 70% after 42 days of biodegradation, but their intermediates obtained by partial oxidation ( products of hydrolytic-hydroxylation) and/or a product of their partial reductive dechlorination ( phenol) show increased biodegradability. To test the efficiency of the method, water exposed to this contaminant was treated in the laboratory in batch conditions. Because the products of partial oxidation and partial reductive dehalogenation of 4-chlorophenol essentially differ, the main factor studied was the efficiency of biodegradation of 4-chlorophenol after oxidative or reductive pre-treatment steps. In comparison with the rate of biodegradation using free cells without application of the pre-treatment step, the rate of degradation of 4-chlorophenol by the application of consecutive combination of Fenton reagent and biotreatment was two-fold. As for the combination of reductive dechlorination pre-treatment step with consecutive biodegradation, the rate of decontamination of the 4-chlorophenol was a little bit higher here in comparison with the rate of biotreatment after the pre-oxidizing step: the remaining concentration of 4-chlorophenol corresponding to the sampling in 7, 28, and 56 days after the inoculation were 70 mg/L, 12 mg/L, and 1.1 mg/L, respectively, in samples containing the average initial concentration of 126 mg/L of 4-chlorophenol. Positive results may probably be due to the co-substrate effect of phenol presented in the samples after the pretraetment reductive step. We have shown that both procedures followed with aerobic biodegradation can be considered suitable for removing hazardous chlorinated compounds from contaminated waters. The rate of biodegradation after the application of pre-treatment procedures was slightly enhanced in comparison with the rate of biodegradation without the application of the pre-treatment steps. It is evident that the choice of the decontamination pre-treatment procedure cannot be generalized and will essentially depend upon the type and concentration of target contaminants and on process costs.