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Introduction: Phenolic compounds are ubiquitous compounds found in all plants as their secondary metabolites. Phenols are becoming increasingly important particularly because of their beneficial effects on health., Objective: To provide a faithful calibration model for the simultaneous determination and quantification of phenolic acids, as salicylic, vanillic, p-hydroxybenzoic acids, eugenol and thymol in different extracts of medicinal plants, a comparative study was made between two methods of infrared measurements based on attenuated total reflectance (ATR) and transmission., Methods: Characteristic absorbance peak heights of mid-infrared spectra of individual phenolic acids were measured for the compounds. For partial least squares regression (PLS-R) calibration mixtures of phenolic acids, wavenumber ranges, spectra pretreatment and number of latent variables, were assayed to improve the prediction capability of models using different spectral preprocessing techniques after mean centring of infrared data. Plant extracts were prepared by using water/methanol and ethanolic extraction solvents followed by Fourier-transform infrared (FTIR)-spectrometry analysis. The concentrations of phenolic compounds contained in the extracts were obtained by using the best models selected of the PLS calibration., Results: PLS-ATR-mid-infrared (MIR) measurement provided the most accurate results and offers a good methodology for the determination of phenolic acids. The analysis showed that the rate of phenolic acids and monoterpenic phenols in extracts of medicinal plants is in the same range obtained with the Folin-Ciocalteu method, which confirm that the developed method using PLS is therefore, highly specific and selective., Conclusion: The simultaneous direct quantification of various phenolic acids in different plant extracts was possible with a fast and simple methodology based on PLS-ATR-FTIR analysis., (© 2020 John Wiley & Sons, Ltd.)