Abstract: High-field I3C NMR of propyllithium-l-13C-I-6Li (4) at equilibrium in cyclopentane shows splitting due to I3C-6Li spin coupling, and reveals the existence of several organolithium species with aggregation numbers of 6, 8, and 9 whose distribution varies with temperature and which all undergo fast intraaggregate carbon-lithium bond exchange. Equilibrium constants and thermodynamic data connecting the aggregates are given. ...