This research investigates the influence of Ultra-High Temperature Ceramic (UHTC) additives on the microstructure and oxidation behavior of Ceramic Matrix Composites (CMCs) synthesized via Reactive Melt Infiltration (RMI). The study focuses on three key parameters of the UHTC additives: particle size, additive volume, and chemical composition. Comprehensive material characterization was conducted throughout the RMI process to study the relationships between these parameters and the resulting microstructural quality. The synthesized samples were subjected to furnace oxidation tests at 1000°C and 1200°C over varying exposure durations. The experimental results demonstrated the oxidation response of ZrB2- and ZrC-based UHTCMC materials produced by RMI, providing insights into optimizing additive selection and processing conditions for improved (ultra)-high temperature application.