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In the Mamais-Jenkins-Pitt method for determination of readily biodegradable COD (SS), 2 alternatives were proposed for the intermediate determination of soluble inert COD (S1). When a full-scale treatment plant exists, influent S1 = effluent truly soluble COD. When there is no full-scale plant, then the truly soluble COD of the effluent of a 24 h fill-and-draw batch reactor treating the wastewater is taken as influent S1. In this study, both S1 methods were statistically compared on 24 wastewater samples from 2 municipal wastewater treatment plants (WWTPs). While average S1 obtained for the 2 methods was the same, individual samples usually had very different S1 values. In fact, virtually no correlation was found between the 2 methods. Also, the SS values obtained using both S1 alternatives were statistically compared. A good correlation was observed, in spite of the poor S1 correlation - low, dispersed SS values did not seriously affect the correlation between both SS determinations. A method was proposed for determination of the limit of detection and the limit of quantification (LOQ) for both SS methods. The LOQ resulted in 28.6 mg/ℓ and 32.6 mg/ℓ, respectively, for the full-scale and the laboratory-scale alternatives. Some assumptions of the original laboratory-scale (LS) method could potentially be sources of error in S1 determination. Two modifications to the laboratory-scale method were implemented in order to avoid these potential problems: Washing biomass with tap water, and correcting S1 in the fill-and-draw reactor by the SS of the original biomass suspension. These method modifications were tested on wastewater samples from the mentioned WWTPs. The fundamentals and results of both modifications are discussed in this paper, as well as the imprecision associated with estimating influent S1 from effluent CODsol in all studied methods, and its impact on SS determination. [ABSTRACT FROM AUTHOR]