Non-isothermal siliconizing in Si-clad Mo wires has been studied over a wide range of heating rates Vh and silicon layer thicknesses δSi. At T = 1000–1600°C, the reaction was found to proceed in two stages: by Mo(s) + Si(s) mechanism (stage I) and by Mo(s) + Si(l) mechanism (stage II). For sufficiently thin silicon layers, non-isothermal Mo-Si reaction is single-step and ends up before reaching the melting point of silicon: a decrease in the heating rate is accompanied by marked expansion of a range of Si layer thickness (δSi) within which the Mo-Si reaction accomplishes at stage I. Based on the above data, the Vh-δSi diagram discerning four different siliconizing modes of Mo was plotted. Within a wide range of heating rates and for sufficiently small Mo particles non-isothermal reaction of Mo with Si may proceed by purely solid-state mechanism.