Technical Abstract: The in vitro antimicrobial effect of cinnamaldehyde and Sporan in combination with acetic acid against E. coli O157:H7 and Salmonella was investigated. A five strain cocktail of E. coli O157:H7 and Salmonella were inoculated in Luria-Bertoni broth (LB broth, 7 log CFU ml-1) containing cinnamaldehyde or Sporan (800 and 1000 ppm) alone or in combination with 200 ppm acetic acid, and incubated at 37°C for up to 6 h. Surviving populations of test pathogens were determined every h by spiral plating on XLT4 and Sorbitol MacConkey Agar for Salmonella and E. coli O157:H7, respectively. Bacterial cells treated with oils were observed under electron microscopy to determine the mode of antimicrobial action. E. coli O157:H7 and Salmonella were undetectable after 1 h in presence of 800 ppm cinnamaldehyde. A 1000 ppm Sporan significantly reduced Salmonella and E. coli O157:H7 populations by 1.83- and 3.02 log CFU ml-1 within 2 and 4 h, respectively. The synergistic effect of acetic acid was not evident as it did not enhance (P > 0.05) the bactericidal activity of oils. Scanning and Transmission electron microscopy of oil-treated bacterial cells revealed cell structural damage and leakage of cellular content. Cinnamaldehyde was highly effective against both E. coli O157:H7 and Salmonella whereas the effect of Sporan was dependent on its concentration, exposure time, and pathogen. E. coli O157:H7 was more sensitive to the oils than Salmonella. Cinnamaldehyde and Sporan may be effective in reducing or eliminating the growth of E. coli O157:H7 and Salmonella in foods.