all rights reserved Microfibril angle (Mfa) is a property of the cell wall of wood fibres, which is made up of millions of strands of cellulose called microfibrils. 1 this elementary wood trait represents the orientation of crystalline cellulose in the cell wall with respect to the stem axis. 2 It is of particular interest for breeding programmes 3,4 since Mfa has major effects on two key properties of wood: its stiffness and longitudinal shrinkage. 5 among all available techniques, only X-ray diffractometry (Xrd) provides quick Mfa measurements for a large number of samples; 6,7 however, sample preparation is often time-consuming. Xrd has been largely used because of the crystalline arrangement of cellulose microfibrils in the wood cell wall; it allows a study of not only its organisation (such as Mfa) but also its apparent crystal size 8 or its mechanical state. 9 numerous papers have proposed near infrared (nIr) spectroscopy to determine Mfa (table 1). nIr spectroscopy is a rapid method for the determination of many chemical properties which have been successfully related to physico-mechanical properties of wood. 26 one of its main advantages is the possibility of estimating a range of wood traits from the same nIr spectra. to explain the results of these established nIr-based calibrations for Mfa, a common assumption is that the supposed correlation that exists between a cIrad-perSySt department-production and processing of tropical Woods, 73 rue Jean-françois Breton ta B-40/16, b laboratoire de Mécanique et Génie civil, cnrS, université Montpellier 2, place e. Bataillon, cc 048, 34095 Montpellier, cedex 5, france c cIrad-BIoS department-Genetic diversity and Breeding of forest Species, 73 rue Jean-françois Breton ta B-40/16, 34398 Montpellier, cedex 5, franceThe microfibril angle (MFA) of crystalline cellulose in the wood cell wall along the stem axis has major effects on stiffness and longitudinal shrinkage of wood and is of key importance to timber quality. The aims of this study were: (1) to develop partial least square (PLS) regression models for microfibril angle (measured on tangential sections by X-ray diffraction) based on NIR spectra measured on tangential and on radial surfaces; (2) to develop PLS regression models for MFA based on radial NIR spectra collected from wood surfaces of different quality; and (3) to verify the reliability of these PLS-R models by external validations. T values were recorded by X-ray diffraction on tangential sections while NIR spectra were taken on tangential and radial wood surfaces. PLS-R calibrations for MFA based on tangential NIR spectra were better (r 2 p = 0.72) than those using radial NIR spectra (r 2 p = 0.64). The key role of the chemical components and the effect of surface quality of wood on NIR spectroscopy calibrations are discussed. Considering the differences between experimental conditions, these findings showed the potential of the NIR-based models for predicting MFA in Eucalyptus wood, even using spectra taken from different wood faces and surface qua...