Combining the techniques of molecular beam and vacuum ultraviolet synchrotron radiation photoionization mass spectroscopy, we have measured the ionization energy of NH3 (10.16 +/- 0.02 eV) and the appearance potentials of H+ (18.57 +/- 0.05 eV) and NH2+ (15.75 +/- 0.02 eV). From these data, we have obtained dissociation energy values for D-0(H2N-H) (4.97 +/- 0.05 eV), D-0(H2N+-H) (5.59 +/- 0.02 eV) and D-0(H2N-H+) (8.41 +/- 0.05 eV) and the ionization energy IE(NH2) (10.78 +/- 0.05 eV). High-level ab initio calculations lead to results that are in good agreement with the experimental findings.