Phenyllithium forms a mixture of tetramer and dimer in ether. Complete conversion to dimeric solvates is achieved by the addition of THF, dioxolane, DME, or TMEDA in near stoichiometric amounts. The addition of 2,5-dimethyltetrahydrofuran favors dimer, but tetramer is still detectable at 14 equiv of cosolvent. PMDTA converts PhLi to monomer in ether. In THF, PhLi is a mixture of dimer and monomer. Addition of TMEDA forms a series of complexes, but the dimer/monomer ratio is essentially unaffected. PMDTA and HMPA form monomeric PhLi stoichiometrically. HMTTA and DMPU also result in monomer formation but several equiv are required. 12-Crown-4 shows no spectroscopically detectable complexation of PhLi in THF. All of the cosolvents tested increase the reactivity of PhLi in THF in a test metalation reaction : HMPA and 12-crown-4 show the largest effects, PMDTA is intermediate, and HMTTA and TMEDA result in the least activation. In two selectivity tests, HMPA and 12-crown-4 show a substantially lower selectivity than the other cosolvents. We postulate that a contribution from a highly reactive separated ion pair (SIP) intermediate is responsible for the lower selectivity.