This research focuses on the production and characterization of (Zr,Ti)B-2 solid solution systems. The purpose of this study is to understand how solid solution formation influences the structure, mechanical properties, and oxidation resistance of ZrB2 and TiB2. For this, herein, monolithic ZrB2, TiB2, and (ZrxT1-x)B-2 solid solution systems (x = 0.25, 0.50, 0.58, 0.75) were prepared. Moreover, 1 vol% graphene nanoplatelets (GNPs) were added to two different solid solution systems (Zr-0.(25),Ti0.75B2 and Zr0.58Ti0.42B2) without affecting the complete solid solubility of these systems. All powder mixtures were densified by spark plasma sintering at 1800 degrees C, under 50 MPa with 5 min holding time. The densification, oxidation behavior, microstructure, phases, and mechanical properties (Vickers hardness and indentation fracture toughness) of these samples were systematically investigated. The formation of solid solutions caused an increase in the Vickers hardness and fracture toughness of the samples. Further improvement in mechanical properties and better oxidation resistance were achieved with the addition of GNPs to the solid solution systems.