Tagitinin C, an antiplasmodial compound, identified as one major compound of the subtropical medicinal plant, Tithonia diversifolia, was determined by FT–IR spectroscopy method. The crude ether extracts from aerial parts of the plant were evaporated to dryness and re-dissolved in tetrachloroethylene (C 2Cl 4) before analysis. The magnitude of the absorbance of the very specific CO stretching vibration (ν CO) at 1664.8 cm −1 was exploited in order to quantify tagitinin C. The determination coefficient ( r 2) of the calibration scale was 0.9994, the detection limit was lower than 3 μg ml −1 and the quantification limit was lower than 10 μg ml −1. Recovery values from 100.5 to 101.7% were found for spiked concentration levels from 19.91 to 89.95 μg ml −1. The main characteristics of the curves obtained from the calibration standards and from the standard addition technique were not statistically different (Student t-test) suggesting that matrix effects were negligible. The results obtained for the determination of tagitinin C in the crude ether extract from aerial parts of T. diversifolia by LC and FT–IR spectroscopic method agreed well: 0.76±0.02 and 0.773±0.009, of tagitinin C in dried plant respectively.