Gas phase hydration of adenine, cytosine, thymine, and uracil in supersonic molecular beams is examined by mass spectra and ionization potential measurements. In the beam source, designed for substances of low volatility that readily decompose on heating, a carrier gas (moist argon or pure water vapor) flows through a porous filter packed with the solid sample before exiting via a supersonic nozzle. With the electron impact energy below the ionization potential of water, progressions of cluster ions of the form B-m(+)(H2O)(n) with m = 1-3 and n as high as 15 are obtained. Cytosine, even with modest heating, loses two hydrogens and also undergoes tautomeric conversion to uracil. From appearance potential measurements and calibrations, the ionization potentials for B = adenine or thymine clusters (with m = 1, n = 0-3; m = 1 or 2, n = 0-1 or 2) were determined; relative to the neutral base monomer, hydration by three water molecules decreases the ionization potential by about 8%, or 0.7 eV.