We have fabricated in the laboratory and subsequently deformed crystalline hydrates and partial melts of the water-rich end of the NH3-H2O system, with the aim of improving our understanding of physical processes occurring in icy moons of the outer solar system. Deformation experiments were carried out at constant strain rate. The range of experimental variables was strain rate 3.5¿10-7≤&egr;¿≤3.5¿10-4 s-1, temperature 132≤T≤220 K, pressure 50≤P≤100 MPa, and mole fraction NH3 0 ≤ xNH3 ≤ 0.295. Phase relationships in the NH3-H2O system indicate that water ice and ammonia dihydrate, NH3⋅2H2O, are the stable phases under our experimental conditions. X ray diffraction of our samples usually revealed these as the dominant phases, but we have also observed an amorphous phase (in unpressurized samples only) and occasionally significant ammonia monohydrate, NH3⋅H2O. The onset of partial melting at the peritectic temperature at about 176 K appeared as a sharp transition in strength observed in samples of xNH3 = 0.15 and 0.295. In samples of xNH3 = 0.05 and 0.01, the effect of melt was less pronounced. For any given water ice+dihydrate alloy in the subsolidus region, we observed one rheological law over the entire temperature range from 176 K to about 140 K. Below 140 K, a shear instability similar to that occurring in pure water ice under the same conditions limited our ability to measure ductile flow. The rheological laws for the several alloys vary systematically from that of pure ice to that of dihydrate. Pure dihydrate is about 4 orders of magnitude less viscous than water ice just below the peritectic temperature, but because of a very pronounced temperature dependence in dihydrate (100 kJ/mol versus 43 kJ/mol for water ice) the viscosity of dihydrate equals or exceeds that of water ice at T≤140 K. The large variation in viscosity of dihydrate with relatively small changes in temperature may be helpful in explaining the rich variety of tectonic and volcanic features seen on the surfaces of icy moons in the outer solar system. ¿ American Geophysical Union 1993