The growth yield coefficients of phenol as well as of chloro- and methyl-substituted derivatives and the respective phenoxyalkanoic acids have been considered in a theoretical study. The yield coefficient of phenol assimilated via the ortho pathway is at a given P/O quotient 9-23% higher than that of the meta route. Chlorine, removed by a reductive mechanism, decreases the yield figure by 10% per chlorine atom; hydrolytic mechanisms are more favourable in this respect. Methylation has a positive effect on the energetics of phenol utilization. Phenoxyacetic acid derivatives have a reduced yield per carbon atom in comparison to the respective phenol derivative. This is compensated for by higher chain lengths of the alkanoic acid moiety. The yield coefficients calculated for phenol correspond to a P/O quotient of 2-3 as compared to the experimental carbon conversion. With chlorinated derivatives, e.g. 2,4-dichlorophenoxyacetic acid or pentachlorophenol, the apparent gain in energy reflects a P/O < 1, indicating uncoupling effects during growth on such compounds.