Aims Investigate the interactions of organic acids (OAs), acetic, butyric, citric, formic, lactic and propionic acid against 50 Gram-positive vancomycin-resistant Enterococcus faecium (VRE) strains to determine whether pH, undissociated or dissociated acid forms correlate with bacterial inhibition. Methods and Results Concentrations of undissociated and dissociated OAs at the molar minimum inhibitory concentrations (MIC(M)s) of the VRE were calculated using the Henderson-Hasselbalch equation. The pH at the MIC(M)s of all VRE strains against acetic, butyric, formic and propionic acids was similar, 4.66 +/- 0.07, but there was a 1.1 pH unit difference for all six OAs. Inhibition of VRE by all six OAs did not appear to be solely dependent on pH or on the undissociated OA species. The inhibition of VRE by all six dissociated acids was within Delta = 3.1 mmol l(-1). Conclusions Vancomycin-resistant Enterococcus faecium inhibition correlated with the dissociated OA species. A small decrease in the concentration of the dissociated OAs from optimum may result in allowing VRE strains to escape disinfection. Significance and Impact of the Study When an OA is used to disinfect VRE strains, the concentration of the dissociated OA should be carefully controlled. A concentration of at least 20 mmol l(-1) dissociated OA should be maintained when disinfecting VRE.