Triphenylsilylpotassium (Ph3 SiK). diphenylmethylsilylpotassium (Ph2 MeSiK ), and phenyldimethylsilylpotassium (PhMe2 SiK) react very rapidly with fluorene in tetrahydrofuran (THF). The reactions can be followed at low temperatures using a stop-flow technique. The thermodynamic constants of activation were calculated for the reactions of organosilylpotassium (R3 SiK) with fluorene for —50°. The results were compared with the corresponding reactions of or-ganosilyllithium (R3 SiLi), and organosilylsodium (R, SiNa) with fluorene, also calculated for —50°. The bimolecular rate constant k increases with the increase in rac.lus of cation. The sequence for the 01/1.- and ASt. values, R3 SiNa