This study aims to investigate the extraction of formic acid by a high molecular-weight aliphatic amine, tridodecylamine (TDA), and a phosphorus-bonded, oxygen-containing extractant, tributyl phosphate (TBP), dissolved in five different diluents (ethyl valerate. diethyl adipate, diethyl sebacate, I-octanol, and heptane) each The experiments were carried out at 298 15 K Comparisons of the results were made using the distribution coefficient, loading factor, separation factor, and the percentage of extraction yield. The results demonstrated that combination of TDA and diethyl adipate had the greatest synergistic effects with a distribution value of 6 The maximum value of the extraction efficiency was found to be 87 % with a 1 09 mol . L(-1) concentration of TDA in diethyl Alpine.