Ion exchange in the synthetic zeolite K-F has been examined at 25 °C and in 0·1N solutions for the ion pairs Na Cs, Na K, K Cs, Na Li, K Li, 2Na Ca,2Na Sr, 2Na Ba, and 2K Ba. K-F containing intracrystalline KCl [i.e., K-F(Cl)] has been prepared and the isotherm for salt uptake measured. Cation exchange has been examined in K-F(Cl) with different contents of KCl and it is shown that the presence of the salt within the crystals changes both isotherm contours and ion selectivities. Na-F shows high selectivity for Ca, Sr, and especially Ba, and should be useful in removing such ions from solution. Exchange isotherms have been successfully interpreted by assuming two kinds of site for the exchanging ions, and in certain of the exchange reactions thermodynamic equilibrium constants have been obtained. Immiscibility gaps were observed with Na-F and K-F and with Na-F and Li-F, each such gap being associated with a hysteresis loop. Confirmation of the two-phase region was obtained by X-ray powder diffraction.