The ionization energies of complexes between a rare-earth metal (Yb) and ammonia were measured for the first time. Using photoionization mass spectrometry under molecular-beam conditions, the adiabatic ionization energies of Yb(NH3)n were determined for n=1-10. Ab initio calculations were also carried out in support of this work and were found to be in excellent agreement with experiment. The combined findings from theory and experiment are consistent with formation of "interior" complexes in which the Yb atom is embedded within a shell of NH3 molecules, rather than sitting on the surface of an (NH3)n cluster. The calculations also suggest that Yb can accommodate up to eight NH3 molecules in its first solvation shell before steric repulsion makes occupancy of the second solvation shell more favourable energetically. The experimental ionization data are consistent with this prediction, as demonstrated by below-trend adiabatic ionization energies for the n=9 and 10 complexes. The ionization energies of Yb(NH3)n complexes closely follow those for complexes of alkali metal atoms with NH3, which suggests that a valence electron will eventually detach from the Yb atom to form a solvated electron in Yb(NH3)n when n is sufficiently large.