A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Al–Si ternary system which is very valuable for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Aluminum is an alloying element for these composites, thus the Nb–Al–Si phase diagram, especially solubility of Al in Nb5Si3, is important information for the composite design. An isothermal section at 1000 °C was constructed from the results obtained from a diffusion multiple using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A ternary phase Nb3Si5Al2 was observed. The solubility data of Al in αNb5Si3 and NbSi2 as well as Si solubility in Nb3Al, Nb2Al and NbAl3 were obtained. The new isothermal section helps to judge the reliability of the existing literature results and to add new data to the Nb–Al–Si phase equilibria.