On the nanometric scale, alloy nanomaterials behave as new materials because they are able to display structures and properties that are different from those of pure metal nanostructures or bulk alloys. In this work, the compositional characteristics of AlCu alloy nanoparticles (NPs) obtained by Femtosecond Laser Ablation Synthesis in Solution (FLASiS) on solid targets with different percentages of Al and Cu are studied, motivated by their increasing importance in the construction, aerospace, and transportation industrial areas (stronger resistance to corrosion), catalytic activity and antibacterial effects. The morphology, structure, composition, and plasmonic behavior of the NPs are analyzed through scanning electron microscopy, transmission electron microscopy (TEM), energy‐dispersive spectroscopy, X‐ray diffraction, and optical extinction spectroscopy. A configuration model of a host alumina matrix with Cu inclusions and a possible kinetic formation mechanism of these nanostructures are both proposed for the first time. Analysis of the plasmonic resonance in the experimental optical extinction spectra shows that the NPs in the colloid have Cu filling factors in the range of 0.002–0.005. Results from TEM images on selected NPs are compared with those obtained independently from the fitting of experimental spectra using the proposed model. Deviation from bulk stoichiometry in the alloy NPs is discussed.