The uniaxial stress components (USC), t, have been measured in NaCl samples, under variable pressure and temperature, in tungsten carbide (WC) toroidal anvils by energy dispersive X-ray diffraction under synchrotron radiation in a large volume Paris-Edinburgh cell. It was observed that t increases with load and at p= 4.6 GPa, t = —0.6 GPa (measured from the mean value of the lattice parameter of NaCl). When heating is applied (under load), t decreases, and becomes zero for T < 600 K. On cooling at constant load, t remains negligible (t = 0.01 GPa) down to room temperature. After loading again (p = 3.8 GPa) at room temperature, t remains small (t = —0.07 GPa). A measurement of the FWHM of the diffraction peaks verifies the improvement in hydrostaticity after heating.