Arm-growth initiation in six-armed Z-RAFT star polymerization of styrene-d(8) was followed via the disappearance of H-1 NMR signals of the leaving group at their original position. These measurements revealed a dramatic impact of the leaving group on star polymer topology. Phenylethyl as leaving group induced formation of completely initiated stars already after 8% of monomer conversion. Benzyl as leaving group, however, drastically slows down arm-growth initiation, resulting in fully initiated star polymers only after about 60% of monomer conversion. Below that value, star-polystyrene of the expected topology cannot be generated with benzyl as leaving group. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] Published online 25 September 2008