2-Ethylhexyl phosphonic acid mono-2-ethylhexyl ester(PC-88A) is an established extractant for rare earth separation; however, the influence of tri-n-butyl phosphate (TBP) in PC-88A-based extraction of La(III), Nd(III), and Pr(III), particularly in sulfate medium, has not been previously investigated. In this study, the effects of equilibrium pH, PC-88A concentration (10-30%, v/v) and PC-88A:TBP ratio (1:1-4:1) on the distribution coefficients and separation factors of the three lanthanides were systematically examined, and the role of TBP was further investigated by slope analysis and FT-IR spectroscopy. Increasing pH and PC-88A concentration both enhanced extraction, with DNd(III) and DPr(III) reaching 1.39 and 1.02 at 30% PC-88A(v/v) and pHeq 2.10, while DLa(III) remained negligibly low (0.05), yielding β(Nd/La) and β(Pr/La) of 27.05 and 19.81, respectively. In contrast, increasing TBP concentration consistently suppressed extraction for all three lanthanides, with negative slope values of -0.97, -0.47 and -0.35 for La(III), Nd(III), and Pr(III), respectively, demonstrating that TBP acts as an antagonist rather than a synergist. FT-IR spectroscopy revealed a substantial shift in the P=O stretching band of PC-88A (from ~1198 to 1270 cm⁻¹) exclusively at the 1:1 PC-88A:TBP ratio, consistent with the antagonistic behaviour established by slope analysis. These findings suggest that controlling the PC-88A:TBP ratio offers a practical means of tuning La/Nd and La/Pr selectivity, and provides the first systematic account of TBP's antagonistic role in a PC-88A-based extraction system in sulfate medium.