Fibre-reinforced polymers (FRP) have a good balance between strength, toughness, weight, anti-corrosion and price. They are used to save energy, materials and costs in energy-generation, transportation, construction and others ^{1}. Currently, their low-weight advantage is partlymitigated by alack of confidence in their mechanical behaviour ^{2}. This calls for a better understanding of the micro-mechanics of the material^{3} which is reflected in- and improved by how models represent phenomena.Characterisation, a required step for materials’ selection andfurther design, includes the identification of constitutive parameters ^{4}. The latter is frequently based on inverse simulations using experimental results to construct a loss function. The progress in imaging analysis now allows such a function to be linked to full fields of 3D deformation frommicro- and nano X-ray tomography, e.g. through digital volume correlation (DVC). This information can then be used to adjust the parameters of micro-mechanical models by computer-based simulations ^{5,6}. Here, a full field of 3D deformation from X-ray tomography and [DVC] are combined to determine in-situ parameters of micro-mechanical models of [FRP]. This approach is based on gradient-based optimization and a fast-fourier transform spectral solver to fit the (matrix) parameters of a well-established elasto-plastic model ^{7} for different fibre-volume ratios. To study the feasibility of the approach, a benchmark which is based on synthetic [FRP] microstructures has been set up. The results show that the combination of [DVC] and full-field 3D simulations provides a mean for parameter identification of in-situ properties of [FRP]. 3 References ════════════ 1. Harris, B. /Fatigue in composites/. (Woodhead Publishing Limited, 2003). 2. Breite, C. [Aligning Fibre Break Models for Composites with the Observable Micro-Scale Material Behaviour]. (2021). 3. Huang, S., Fu, Q., Yan, L. & Kasal, B. [Characterization of interfacial properties between fibre and polymer matrix in composite materials - A critical review]. *13*, 1441–1484 (2021). 4. Chevalier, J. [Micromechanics of an epoxy matrix for fiber reinforced composites : experiments and physics-based modelling]. 5. Buljac, A. /et al./ [Digital Volume Correlation: Review of Progress and Challenge