We have investigated structural transitions in Pr2NiO4+delta (delta greater-than-or-equal-to0) using DSC, TGA and XRD measurements. This study reveals firstly that the so called LTT (Low Temperature Tetragonal) phase i.e. P4(2)/ncm symmetry exists not only for delta=0 but also for slightly sur-stoichiometric compounds. The transition temperature from P4(2)/ncm to Bmab is independent on delta in the small range of oxygen excess allowed. Moreover, an orthorhombic-tetragonal transition at about 720 K in the oxidized Pr2NiO4+delta was emphasized and extensively discussed. This transition occurs simultaneously with an oxygen loss. It is shown that the parameter controlling this transition is neither the temperature nor the oxygen partial pressure but the value of delta. This structural change occurs for the threshold value delta(c) = 0.192. We suggest that this threshold reflects a frontier between an oxygen disordered HTT (High Temperature Tetragonal) phase and an orthorhombic phase LTO (Low Temperature Orthorhombic) in which excess oxygen and/or holes might be ordered. Finally, general considerations on the stability of this K2NiF4 type oxide and evolution of transition temperatures have been done as a function of the rare earth ion size.