More than 70 equilibrium constants K between acids and bases, mainly phosphine derivatives, have been measured in tetrahydrofuran (THF) at 20 degrees C by H-1 and/or P-31 NMR. The acids were chosen or newly synthesized in order to cover the, wide pK(a)(THF) range of 5-41 versus the anchor compound [HPCy3]BPh4 at 9.7. These pK(alpha)(THF) values-are approximations to absolute, free ion pK(a)(THF) and are obtained by crudely correcting the observed K for 1:1 ion-pairing effects by use of the Fuoss equation. The acid/base compounds include 14 phosphonium/phosphine couples, 17 cationic hydride/neutral hydride couples, 9 neutral polyhydride/anionic hydride couples, 14 dihydrogen/hydride couples, and 4 other nitrogen- and phosphorus-based acids. The effects on pK(alpha) of the:counterions BAr'(-)(4) and BF4- vs BPh4- and [K(2,2,2-crypt)](+) versus [K(18-crown-6)](+) are found to be minor after correcting for differences in inter-ion distances in the ion-pairs involved. Correlations with nu(M-H) noted here for the first time suggest that destabilization of M-H bonding in the conjugate base hydride is an important contributor to hydride acidity, It appears that Re-H bonding in the anions [ReH6(PR3)(2)](-) is greatly weakened by small increases in the basicity of PR3, resulting in a large increase in the pK(alpha) of the conjugate acid ReH7(PR3)(2) Correlations with other scales allow an estimate of the pK(alpha)(THF) values of more than 1000 inorganic and organic acids, 20 carbonyl hydride complexes, 46 cationic hydrides complexes, and dihydrogen gas. Therefore, many new acid-base reactions can be predicted and known reactions explained. THF, with its low dielectric constant, disfavors the ionization of neutral acids KA over HB+, and therefore separate lines are found for pK(alpha)(THF)(HA) and pK(alpha)(THF)(HB+) when plotted against pK(a)(DMSO) or pK(a)(MeCN). The crystal structure of [Re(H)(2)(PMe3)(5)]BPh4 is reported.