The Escherichia coli single-strand binding (SSB) protein binds single-stranded (ss) nucleic acids in at least four distinct binding modes depending on the salt conditions [Lohman, T. M., & Overman, L. B. (1985) J. Biol. Chem. 260, 3594; Bujalowski, W., & Lohman, T. M. (1986) Biochemistry 25, 7799]. Equilibrium binding constants for the interaction of the E. coli SSB protein with poly(A), poly(U), poly(d A), and poly(dT) have been measured over a range of monovalent salt concentrations and types under conditions which favor only the high site size, (SSB)65binding mode, which covers 65 nucleotides per SSB tetramer. The binding isotherms are analyzed by using a statistical thermodynamic model (“tetramer/octamer” model) that assumes cooperative binding of SSB is limited to the formation of octamers [Bujalowski, W., & Lohman, T. M. (1987) J. Mol. Biol. 195, 897] rather than the indefinite clustering of tetramers. The dependence of the intrinsic association equilibrium constant, Kobsd, and cooperativity parameter, ωT/O, on salt concentration has been determined by titrations which monitor the fluorescence quenching of the SSB protein upon complex formation. In the (SSB)65binding mode, SSB binds with only moderate cooperativity to ss nucleic acids [Lohman, T. M., Overman, L. B., & Datta, S. (1986) J. Mol. Biol. 187, 603]. The cooperativity parameter derived from the tetramer/octamer model, which represents the equilibrium constant for formation of a nucleic acid bound SSB octamer from two nucleic acid bound tetramers, has a value of ωT/ O= 410 ± 120 and is independent of salt concentration and type for poly(dA), poly(U), and poly(A) (25 °C, pH 8.1). However, Kobsddecreases steeply with increasing salt concentration, such that ∂ log Kobsd/∂ log [NaCl] = -7.4 ± 0.5 for poly(U), -6.1 ± 0.6 for poly(dA), and -6.2 ± 0.3 for poly(A) (25.0 °C, pH 8.1). The SSB-poly(dT) affinity is too high to measure in buffers containing even 5 M NaCl; however, in 1.8-2.5 M NaBr, we measure ∂ log Kobsd/ϑ log [NaBr] = -5.7 ± 0.7, with a lower value of ωT/O= 130 ± 70. The polynucleotide specificity of the (SSB)65binding mode (0.20 M NaCl, 25.0 °C, pH 8.1) is KObsd(dT) > Kobsd(dC) » Kobsd(ss M13 DNA) > Kobsd(I) > Kobsd(U) = 8Kobsd(dA) = 87Kobsd(A) » Kobsd(C). A dramatic effect of anion type on both the salt dependence and magnitude of Kobsdis also observed such that for poly(U) ∂ log Kobsd/∂ log [potassium glutamate] = -5.7 ± 0.4, ∂ log Kobsd/∂ log [NaF] = -4.3 ± 0.4, ∂ log Kobsd/∂ log [NaCH3COO] = -6.5 ± 0.2, and ∂ log Kobsd/∂ log [NaBr] = -6.7 ± 0.6; in 0.35 M monovalent cation, Kobsd(Glu) = 5.6Kobsd(F) = 11Kobsd(CH3COO) = (1.1 X 103)Kobsd(Cl) = (1.1 X 104)Kobsd(Br). These data indicate that significant electrostatic interactions occur in the (SSB)65complexes formed with all polynucleotides, resulting in a net release of both cations and anions, although there are also contributions due to cation and anion uptake.