A set of experiments on mechanical alloying of Cu and Y powders of two compositions is performed in a high-energy planetary mill in air. The first composition consists of equiatomic Y–Cu mixture, while the second composition contains an admixture of 1% vol. of multiwalled carbon nanotubes (CNTs). The synthesis products of Y–Cu and Y–Cu–CNT mixtures, selected after each 10 min of processing, are studied by a combination of X-ray diffraction methods (qualitative and quantitative phase analysis, refinement of the crystal structure of individual phases). Based on the data obtained, the effect of atmospheric oxygen and carbon dopants (CNTs) on the phase composition and the crystalline structure of reaction products are analyzed. As shown, the oxidation of reaction products is accompanied by the formation of a mixture of oxygen-containing phases, namely, Y$_2$O$_3$/Y$_{1–x}$Cu$_{x}$O for the Y–Cu composition and Y$_2$O$_3$/Y$_{1–x}$Cu$_{x}$O$_{0.5}$C$_{0.5}$ for the Y–Cu–CNT composition. Moreover, it is established that Y$_{1–x}$Cu$_{x}$O oxide and Y$_{1–x}$Cu$_{x}$O$_{0.5}$C$_{0.5}$ carboxide (both crystallized in a NaCl type structure) are the substitutional solid solutions, the copper content of which increases to 25 at.%. As shown, the phase transformations during Y–Cu mixture processing occur 10 min earlier than similar transformations during Y–Cu–CNT mixture processing, which means that the carbon dopants (in form of CNTs) could be an oxidation inhibitor at mechanochemical synthesis.