Abstract: Reactivity of nanosized objects is a hot topic in modern solid-state chemistry and materials science. The present work is focused on the interaction between multi-walled carbon nanotubes (MWCNTs) and metallic titanium during high-energy ball milling and thermal explosion, a rapid temperature rise in a mixture caused by an exothermic reaction ignited by external heating and occurring throughout the sample volume. A fundamental property of the nanocomposite mixture—the ability of its components to react in the combustion mode—is explained; an analysis of the dependence of the combustion characteristics of the nanocomposites on the milling duration of powder mixtures is provided. The phase and structural transformations of the Ti-MWCNT mixtures have been analyzed using X-ray diffraction and transmission electron microscopy. It was found that the ball-milled powders contain nanostructured titanium, nanotube fragments, amorphous carbon and nanosized carbon-deficient titanium carbide TiC x . Within the nanocomposite powder particles, TiC x nanoparticles are covered with layers of amorphous carbon. Thermal explosion was observed in Ti-4mass%MWCNT mixtures milled for 1.5–7 min. Shorter milling times were apparently not sufficient for establishing a proper interfacial contact, while longer milling times led to the extensive formation of titanium carbide TiC x , which acted as a barrier lowering the heat release by the mixture upon ignition. Both the ignition temperature of Ti-4mass%MWCNT and the maximum temperature developed during thermal explosion decrease with the milling time. A comparison of the behavior of MWCNT with that of carbon black under conditions of thermal explosion in the mixtures with titanium is also presented. Graphical abstract: [Figure not available: see fulltext.].