Investigations were carried out to assess the efficiency of four plant essential oils; bay, clove, cinnamon and thyme as natural food preservatives. The effect of the plant essential oils at concentrations of 0.1, 0.5 and 1% was studied in low-fat and full-fat soft cheese against Listeria monocytogenes and Salmonella enteritidis at 4° and 10°C respectively, over a 14-day period. The composition of the cheese was shown to be an important factor in determining the effectiveness of the plant essential oils. In the low-fat cheese, all four oils at 1% reduced L. monocytogenes to = 1.0 log10 cfu ml-1. In contrast, in the full-fat cheese, oil of clove was the only oil to achieve this reduction. Oil of thyme proved ineffective against S. enteritidis in the full-fat cheese, yet was equally as effective as the other three oils in the low-fat cheese, reducing S. enteritidis to = 1.0 log10 cfu ml-1 from day 4 onwards. It is concluded that selected plant essential oils can act as potent inhibitors of L. monocytogenes and S. enteritidis in a food product. © 2001 Academic Press.