Titanium carbonitrides (TiCN) are very hard materials with increasing technological importance. They are mainly applied in composites with various metal carbides and/or metallic binders for cutting tools. In these applications, high temperatures are usually involved and, sometimes, chemical reactions. Being composed of a metallic-binder phase, in which a hard ceramic phase is dispersed, the performance of these materials can be largely affected by temperature and environment. In this work, nearly fully-dense TiCN based cermets have been produced starting from the following composition (wt%): TiC0.5N0.5+15.3(WC-5%Co)+6.2Ni+2.1Co. Some of the mechanical properties have been routinely measured, such as hardness, Young modulus, fracture toughness and flexural strength. The main effort has been devoted to the study of the effects of temperature and environment on the fracture toughness. Chevron notch tests have been carried out at room temperature, 600 °C, 800 °C and 1 000 °C in static air, flowing air and flowing nitrogen atmosphere. The influence of the interaction between temperature, environment and microstructure on the fracture behavior has been analyzed.