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Light emitting diodes (LEDs) are known to inactivate foodborne pathogens. However, the effect of food acidulants on the antibacterial effect of LEDs is unknown. In this study, the influence of organic acids on the photodynamic inactivation of four foodborne pathogens (Escherichia coli O157:H7, Salmonella Typhimurium, Listeria monocytogenes and Staphylococcus aureus) by blue LEDs was investigated. Each pathogen was illuminated by 10-W 461 nm LEDs (22.1 mW/cm²) at 15 °C for 7.5 h in tryptone soya broth of pH 4.5 adjusted with citric, lactic or malic acids. The total energy dosage imparted by the LEDs over the course of the illumination was 596.7 J/cm². Our results showed that at the same pH, the nature of the acids significantly influenced the bacterial inactivation due to the LEDs, irrespective of the bacterial strain. Lactic acid was found to be the most effective in aiding the photodynamic inactivation of the …