Low-temperature rapid injection NMR (RINMR) experiments were performed on two lithium reagents, n-butyllithium and 2-methoxy-6-(methoxymethyl) phenyllithium ( 5), with the goal of measuring the relative reactivity of the different aggregates ( dimer, mixed dimer, and tetramer for n-BuLi, monomer and tetramer for 5) toward typical electrophiles. The reaction of the n-BuLi dimer with ( trimethylsilyl) acetylene first forms the mixed dimer n-(BuLiMe3SiC)-Me-. CLi, which is about 1/60 as reactive as the n-BuLi homodimer. The tetramer does not react. In the deprotonation of (phenylthio) acetylene, the n-BuLi dimer was found to be 3.5 x 10(8) as reactive as the tetramer, and in the addition to p-diethylaminobenzaldehyde, the relative reactivity was at least 2 x 10(4). In the deprotonation of (p-tolylsulfonyl) acetylene, the monomer of 5 was at least 1014 times as reactive as the tetramer. These measurements show that the difference in reactivity between the lower and higher aggregates of organolithium reagents can be many orders of magnitude higher than all previous estimates.