We have investigated the synthesis, structure, and electronic properties of (NH3)xK3C60 by exposing preformed K3C60 to NH3 vapor. Measurements were made on bulk powders and films with a variety of techniques including in-situ x-ray diffraction and monitoring of NH3 pressure, Raman scattering and ac susceptibility. The reaction of NH3 with K3C60 is completely reversible and leaves the (C60)3- charge state intact, while allowing the distance between the C60 molecules to vary. We observe two different crystallographic structures at x=1 and x=8-10. The x=1 structure is face-centered orthorhombic with a single NH3 on the octahedral interstitial site. It is single phase at 100°C and NH3 pressures 400 torr and remains single phase at 0 torr if the temperature is reduced to 25°C. Although (NH3)K3C60 exhibits no superconductivity at ambient pressure, superconductivity can be induced by hydrostatic pressure with an onset at 28 K. The details of the x=8-10 structure are uncertain, however it appears that this phase is body-centered tetragonal with NH3 on both the octahedral and tetrahedral sites. It occurs at room temperature and NH3 pressures 500 torr, but is not stable at lower NH3 pressures. Resistivity measurements of NH3-doped K3C60 films show that the formation of the new structure results in a rapid increase of the resistivity to values exceeding 1 cm.