To better understand the electrostatic component of the interaction between proteins and the polyanion heparin, we have investigated the thermodynamics of heparin binding to positively charged oligopeptides containing lysine or arginine and tryptophan (KWK-CO2 and RWR-CO2). The binding of these peptides to heparin is accompanied by an enhancement of the peptide tryptophan fluorescence, and we have used this to determine equilibrium binding constants. The extent of fluorescence enhancement is similar for both peptides, suggesting that the tryptophan interaction is similar for both. Titrations of these peptides with a series of simple salts suggest that this fluorescence enhancement is due to the interaction of tryptophan with sulfate moieties on the heparin. Equilibrium association constants, Kobs (M-1), for each peptide binding to heparin were measured as a function of temperature and monovalent salt concentration in the limit of low peptide binding density. At pH 6.0, 25 °C, 20 mM KCH3CO2, Kobs = 3.2 (±0.3) x 103 M-1 for KWK-CO2 binding, whereas Kobs = 4.5 (±0.5) x 103 M-1 for RWR-C02. However, the dependence of Kobs on KCH3CO2 concentration is the same for both oligopeptides, each of which possesses a net charge of +2 at pH 6.0. The logarithm of Kobs is a linear function of the logarithm of [KCH3CO2] over the range from 12 mM ≤ KCH3CO2 ≤ 30 mM (pH 6.0, 25 °C), with (δ log Kobs/δ log [KCH3CO2]) = -2.0 ± 0.3, indicating that ~2 ions are released per bound peptide upon formation of the complex. The van't Hoff ∆H°obs for each peptide-heparin interaction is independent of [KCH3- C02], with ∆H°obs = -1 ± 1.5 kcal/mol for KWK-C02, and∆H°obs = -3.5 ± 1.5 kcal/mol for RWR-CO2. Comparison of these results with similar studies of the binding of these same peptides to singlestranded polynucleotides indicates that binding of these peptides to heparin at low salt concentrations is largely driven by the favorable increase in entropy resulting from the release of ions, presumably K+ from the heparin. The results from these model peptide studies are compared with similar studies of protein-heparin interactions.