The invention relates to a method for forming a protective coating against high-temperature oxidation on a surface of a refractory composite material based on silicon and niobium, wherein chromium present on the surface to be protected is reacted with a reactive gas which contains silicon and oxygen in order to produce a composite coating having two phases, a first phase of which is an oxide phase based on silica which has viscoplastic properties and a second phase of which is based on silicon, chromium and oxygen, and wherein the first phase and second phase are coalesced at high temperature, which allows a protective coating to be formed in which the second phase acts as a reservoir to reform, during operation, the first phase by means of reaction with an oxidising gas. The invention is preferably used in the field of aeronautical engines.