Grain refinement in Al-Si alloys is crucial for enhancing material castability and mechanical properties. Industrial practice involves adding inoculants, composed of TiB2 particles coated with metastable Al3Ti via adsorption, to the melt. This introduces essential free titanium for metastable phase formation and subsequent growth restriction. The superstoichiometric grain refiner Al-5Ti-1B, with 2.2 wt.% free titanium, is applied for this purpose. A peritectic reaction forms α-aluminium from this layer. However, when silicon content exceeds 3.5 wt.%, grain coarsening occurs due to silicon’s detrimental effect. This study quantified silicon poisoning in an Al-10Si alloy using stoichiometric and superstoichiometric grain refiners through ASTM-standardized TP1 tests. Adding 0.02 wt.% tantalum acted as an antidote to the τ1 phase, resulting in a finer microstructure. This was attributed to the formation of a Ta-rich layer on TiB2 particles, which causes α-aluminium formation via a peritectic reaction without generating ternary phases with Ti or Si. Correlating to the increasing particle size curves from the TP1 tests, phases were collected in the filter cake with the help of a PoDFA apparatus. These could be examined more closely on the SEM and identified as needle- or plate-shaped. By using an EDS unit, the phases found were assigned to the poisoning phase and further investigated. After the addition of tantalum, a solubility of tantalum could be detected in former poisoning phases. In combination with the gradients of the grain size, it can thus be assumed that tantalum is both an antidote for silicon poisoning in the Al-Si-Ti system and can itself have a grain-refining effect in this system.