Reinvestigation of the cesium enolate (CsPhIBP) of 1-biphenylyl-2-methylpropanone (p-phenylisobutyrophenone, PhIBP) in tetrahydrofuran (THF) solution at 25.0 degrees C shows that its UV-visible spectrum changes with concentration with lambda(max) moving to longer wavelengths in more dilute solution. Analysis of the spectral data over a wide concentration range by singular value determination (SVD) combined with the pK measurements indicates a mixture of monomer (M), dimer (D), and tetramer (T) with equilibrium constants of K-1,K-2 = D/M-2 = 2.89 x 10(4) M-1 and K-1,K-4 = T/M-4 = 7.78 x 10(12) M-3. The pK of the monomer is 25.08. The data are also compared with the corresponding data obtained for the lithium ion pair system reported in the following paper. We find that the cesium ion pair is more highly aggregated and much more basic than the lithium ion pair. The average aggregation number of CsPhIBP at 10(-3) M is 3.2, substantially greater than the value of 2.2 reported previously; the revision arises from our taking into account the concentration-dependent extinction coefficient and absorption band shape of CsPhIBP. The revised value also requires that a correction be applied to our previously reported kinetics of the reaction of CsPhIBP with methyl tosylate (MeOTs); the data indicate that the CsPhIBP ion pair monomer reacts, instead of the free enolate ion that we reported previously. Similarly, alkylation by p-tert-butylbenzyl chloride (BnCl) occurs dominantly via the monomer. More limited studies at -20 degrees C indicate greater aggregation at the lower temperature. Alkylation reactions with MeOTs and BnCl again occur predominantly with the monomer. The reaction products at room temperature are those of C-alkylation with BnCl and equal amounts of C- and O-alkylation with MeOTs.