Grain refinement of Al-Si alloy is an essential method for improving castability and mechanical properties of the material. Industrially, inoculants consisting of TiB2 particles coated with metastable Al3Ti by adsorption, are added into the melt. Free titanium in the melt is necessary for the formation of this metastable phase and the subsequent growth restriction. For this reason, the superstoichiometric grain refiner Al-5Ti-1B, which contains 2.2 wt.-% titanium above the ratio of TiB2 particles, is used. Via a peritectic reaction of this layer, α-aluminium is formed. However, above a silicon content of 3.5 wt.-%, a coarsening of the grains is observed, which occurs due to silicon poisoning effect. This is the formation of the ternary 1 phase (Ti7Al5Si14), whereby a peritectic reaction is no longer possible and thus the grain refinement effect deteriorates. Within the scope of this work, the silicon poisoning of an Al-10Si alloy with stoichiometric and superstoichiometric grain refiner was quantified with by using the ASTM standardised TP1 test. Subsequently, 0.02 wt.-% tantalum was added as an antidote to the 1 phase, resulting in a finer microstructure. The reason for this is the formation of an Al3Ta layer on TiB2 particles, which also forms α-aluminium via a peritectic reaction and has no ternary phases with Ti or Si. In parallel with the TP1 tests, a PoDFA (Porous Disc Filtration Analysis) apparatus was used to collect the poisoning phase in the filter cake and examine it using the scanning electron microscope. After the addition of tantalum to a “poisoned” melt, a solubility of tantalum in the 1 phase with simultaneous refinement of the microstructure were detected.