The softened truss model and the modified compression field theory are two models for the prediction of post-cracking nonlinear response of reinforced concrete (RC) panels subjected to in-plane shear. These models neglect the Poisson's effect of orthogonal compression in the constitutive relationship of the reinforcing bars in tension. This leads to overestimation of the tensile stress in the rebar and consequently shear stress in the panel. To rectify this error, the Poisson's effect needs to be quantified by testing panels under in-plane biaxial tension-compression. A test facility was developed to test reinforced panels under a combination of in-plane biaxial loads. It consists of a self-equilibrating reaction frame and load transferring beams. Panels were subjected to sequential tension and compression to determine the Poisson's effect. The test results show that the Poisson's effect of transverse compression in the longitudinal tensile strain is quite significant.