The commonly used COD parameter does not differentiate between inert and biodegradable organic matter in wastewaters. This differentiation is quite necessary and significant for industrial effluents with high organic content. In such strong wastes the soluble influent COD fraction may severely interfere with the treatability results or challenge the effluent limitations criteria adopted for different industrial categories. The methods suggested in the literature to identify this fraction are not designed to differentiate between soluble inert organic matter and soluble residual microbial products generated during the experiments. This paper proposes two different methods for the assessment of the initial soluble inert COD fraction and summarizes their comparative evaluation. The methods are tested for five different industrial wastes characterizing pulp and paper, meat processing, antibiotics, textile and daily effluents with total soluble COD concentrations raging from 1000 to 9300 mg l-1. The results indicate significant interference of soluble residual microbial products which may be identified and corrected for with the proposed methods.