Technical Abstract: Food-producing animals can be reservoirs of Campylobacter, a leading bacterial cause of human foodborne illness worldwide. Unlike most other gut bacteria, Campylobacter do not ferment carbohydrates but can utilize amino acids as major carbon and energy substrates, a process that can be inhibited by thymol and diphenyliodonium chloride (DIC). In order to determine the effect of these potential additives on feed intake, live weight gain, and gut Campylobacter concentrations, growing pigs were fed standard growing diets supplemented with or without 0.0067 or 0.0201% thymol or 0.00014 or 0.00042% DIC in a replicated study design (N = 4 pens per treatment per replicate, 2 pigs per pen). The diets were offered ad libitum twice daily for 78 days during which time average daily intake (2.39 ± 0.06 kg d-1) were not affected (P > 0.05) by treatment. Pigs treated with DIC, but not thymol, tended (P = 0.07) to have lower rectal Camylobacter concentrations (5.2 versus 4.2 and 4.4 log10 CFU g-1, respectively) or on cecal total cultureable anaerobes (9.8 ± 0.10 log10 CFU g-1). Moreover, effects of treatment were not observed (P > 0.05) on accumulations of the major fermentation end products within collected gut contents. These results suggest that thymol and DIC were appreciably absorbed or degraded in the proximal alimentary tract and that encapsulation technologies will likely be needed to deliver effective concentrations of these compounds to the lower gut to achieve in vivo reductions of Camylobacter.