Experiments were conducted to study the effect of benzoic acid and of essential oil blends in combination with protease on the growth performance of broiler chickens. In the first trial, the birds were divided into three dietary treatments. The control group was fed a basal diet, while the other two groups were given benzoic acid at 300 and 1000 mg · kg^–1, respectively. Growth performance was not affected by benzoic acid inclusion. The pH values of the caecal content decreased following benzoic acid supplementation, while no differences were noticed in the pH of the crop, gizzard, ileum and rectum contents. Following benzoic acid supplementation, lactic acid bacteria populations increased in the caecum, and coliform bacteria, decreased. In the second trial, the birds were divided into three dietary treatments. The controls were fed a basal diet, while the other two groups were given thymol and a mixture of essential oil compounds (30 mg · kg^–1). The dietary inclusion of the mixture of essential oil compounds enhanced growth performance compared with the other groups (P < 0.05), increased lactic acid bacteria populations, and decreased the coliform bacteria population in the caecum. In the third trial, the control group was fed the basal diet, while the other group was given a diet with similar ingredients and containing more benzoic acid and a mixture of essential oils, protease, and less protein and amino acids. In vitro tests showed that addition of benzoic acid, the mixture of essential oils and protease reduced buffering capacity compared with control feed and simulation experiments revealed that the protease increased protein extraction, hydrolysis and digestion. The combination of benzoic acid, essential oils and protease effectively improved weight gain and the feed conversion ratio compared with the control, as well as villus height, lactic acid bacteria counts, and reduced coliform bacteria counts compared with the control group. Finally, it was demonstrated for the first time that the novel, acid-stable protease increases protein solubilization, hydrolysis and digestion in an in vitro simulation model.