Li-6 and N-15 NMR spectroscopic studies of [Li-6,N-15]lithium diisopropylamide ([Li-6,N-15]LDA) and [Li-6,N-15]lithium-2,2,6,6-tetramethylpiperidide ([Li-6,N-15]LiTMP) with [Li-6]LiCl, [Li-6]lithium cyclohexenolate (4), and [Li-6]lithium-2,4-dimethylpentenolate (5) in tetrahydrofuran-hexamethylphosphoramide (THF-HMPA) mixtures are described. LDA-LiCl mixtures show no tendency to form mixed aggregates. LiTMP-LiCl mixtures contain a 1:1 mixed cyclic dimer and a mixed aggregate assigned as an anionic open dimer. LDA-4, LDA-5, LiTMP-4, and LiTMP-5 mixtures all contain 1:1 mixed cyclic dimers as the sob observable mixed aggregate forms. The influence of HMPA on mixed aggregate formation is discussed in light of previous studies of mixed aggregate formation in the absence of HMPA as well as predictions derived from semiempirical (MNDO) computational studies described in the preceding manuscript.