This study was conducted to compare the effects of single and synergistic organic acids (formic (F) and butyric (B) acids) on the performance and lymphoid organs of broiler chicks. In total, 450 one-day-old ISA JA57 chicks were assigned randomly to nine groups, each of five replicates (10 chicks/replicate): The control group, without added acids; groups 2 and 3, with 0.2% and 0.3% formic acid, respectively; groups 4 and 5, with 0.2% and 0.3% butyric acid, respectively; group 6, with 0.2% formic acid and 0.3% butyric acid; group 7, with 0.2% butyric acid and 0.3% formic acid; groups 8 and 9, with 0.2% butyric and formic acids and 0.3% butyric and formic acids, respectively. The control group received tap water, and other treatment groups received acidified drinking water, as previously described. The results indicated that treatment resulted in a significantly higher (p < 0.05) average live weight and weight gain at four weeks of age than in the control group. Nevertheless, group 7 showed significant decreases in the feed conversion ratio compared with the other treatments between four and five weeks of age. The carcass percentage was highest when B3, F3B2, and B2 were added to the drinking water, whereas control and F2B3 showed lower carcass percentage than the other treatments. At 42 days of age, the addition of organic acids to the drinking water of broilers had significant effects on the bursa of Fabricius and thymus percentages, but no effect on the spleen percentage. Water acidification by F and B alone and in combination did not affect poultry performance. However, it improved the lymphoid organ weight, indicating improved immunity and carcass percentage at 42 days of age.