Zirconium-based Metal-Organic Frameworks (MOF) precursors were used to synthesize nanopowders of ZrCxOy zirconium oxycarbide. The reaction mechanisms involved in the thermal transformation of two different MOF precursors obtained by using fumaric (H2FUM) and terephthalic (H2BDC) acid as linkers were investigated. The transformation of these two precursors, characterized by two different C/Zr ratio proceeds through an intermediate zirconia phase formed during heating, in association with turbostratic carbon. The zirconia is shown to nucleate within carbon areas coming from the decomposition of the precursors. Zirconia is mainly observed under its tetragonal form in the Zr-BDC carbon rich system (C/Zr = 8) while the monoclinic form predominates in the Zr-FUM low carbon system (C/Zr = 4). The transformation then finally appears very similar to the classical carbothermal reaction between zirconia and carbon black. The reconstructive transformation of zirconia into an oxycarbide phase being highly energy consuming, high temperature heat treatment are required precluding the possibility to obtain the expected nanocrystalline oxycarbide phase by such a way of synthesis. Coupled XRD and TEM study reveals that Zr-BDC system is considered as carbon saturated and the Zr-FUM system appears as carbon deficient. In the Zr-BDC system, zirconia first disappears during the thermal reaction and the oxygen rich oxycarbide that is first formed then reacts with residual carbon to provide progressive carbon enrichment until the saturation of the ZrCxOy oxycarbide solid solution. On the opposite, in the Zr-FUM system, the carbon is the rate-limiting factor of the reaction and the residual zirconia is shown to react with the oxycarbide earlier formed, suggesting the occurrence of a peritectoid like transformation.