The insulator-metal transition in RNiO3 (R is rare-earth) has revolutionized the fields of condensed matter physics along the last 30 years, aiming to unveil the mechanisms behind the localized-to-itinerant transition in those strong correlated electronic systems. Most studies were devoted to perovskites with 4f R ions; however, little is known on R ions with fully occupied orbitals in the trivalent state, as the case of Tl3+. Here, the main goal is to study the crystal structure evolution across the Néel temperatures (TN = 125 K and TN* = 202 K) and the insulator-metal transition (550–600 K) in TlNiO3 perovskite nickelate using the state-of-art high-resolution synchrotron x-ray diffractometer at D22. The questions to be addressed here: what is the critical temperature for the insulator-metal transition in TlNiO3? Is the its lattice sensible to display magnetoelastic coupling at the reported Néel temperatures? A precise high-temperature phase description of TlNiO3 is highly requested.