The deformation-DIA apparatus (D-DIA) coupled with synchrotron X-rays allows investigating materials elastic and plastic properties at high pressure. Most D-DIA deformation cells use alumina pistons that can also be used for measurement of the differential stress in the compression column by in situ X-ray diffraction. Here, we quantify the axial temperature (T) gradient in the D-DIA deformation cell and better constrain stress measurements in its compression column by studying an alumina specimen compressed and deformed at pressure P in the range 3.9–5.5 GPa and nominal temperature To = 1673 K. The axial T gradient, obtained from alumina equation of state, is ∼155 K/mm at the centre of the cell and does not vary significantly during deformation to 20% specimen strain. This T gradient, if not taken into account when measuring the experimental pressure in the alumina pistons, leads to significantly overestimating pressure. Unlike pressure, stress measurements in alumina are weakly sensitive to temperature. During deformation, the “true” differential stress in the compression column is evaluated at 596 ± 20 MPa using an elastoplastic self-consistent model, while raw uncertainties on experimental differential stresses reach 84 MPa. A comparison between the simulated and experimental data allows to conclude that, although dislocation glide in the basal plane is the primary slip system at run condition, with an estimated critical resolved shear stress (CRSS) of 120 MPa, prism plane slips and pyramidal plane slips also contribute significantly to the aggregate homogenous deformation and texture development, with CRSS on the order of 280 MPa.