This work investigated the toxicity of two isomeric alkylphthalates, i.e., di-n-octyl phthalate (DOP) and di-2-ethylhexyl phthalate (DEHP) to two model bacteria, Escherichia coil (E. coil) and Bacillus subtilis (B. subtilis), which have been previously used to study the toxicity of environmental pollutants. Microcalorimetry was used as the key analytical tool alongside scanning electron microscopy (SEM) and traditional microbiology techniques. The thermokinetic parameters from microcalorimetry showed that the phthalates had a biphasic effect on the metabolic activities of the bacteria; serving as energy sources for the bacteria thereby stimulating their growth at low dosages (<= 150 mu g/mL), but displaying inhibitory effects at higher dosages (>= 300 mu g/mL), indicated by a sharp decrease in growth rate constants at 450 mu g/mL The SEM revealed that the bacterial cells were morphological deformed, with shrunk cells and elongated strands at 600 mu g/mL of both phthalates. The elongated strands inferred that the phthalates inhibited the reproductive processes of the bacteria by possibly impeding some stages of cell division. The half inhibitory concentrations of the phthalates showed that DEHP was more toxic than DOP. Additionally, E. coil, a facultative anaerobe, was more susceptible to the toxic effects of phthalates than B. subtilis, an obligate aerobe capable of forming endospores crucial for tolerating extreme environmental conditions. (C) 2010 Elsevier B.V. All rights reserved.