The steady-state deformation of partially melted ice in the ammonia-water system was studied by means of concentric cylinder viscometer in shear stresses, 10 kPa~0.1 MPa, temperatures, 180--210 K and NH3 contents, 4.0~8.4%. The flow law found was of a non-Newtonian power-law type; the stress exponent was 4.0¿0.1. The activation energy at constant melt fractions was 33.7¿0.8 kJ/mol, which was close to that of viscosity for aqueous ammonia solutions. However, the effective viscosity of partially melted ice estimated at 0.1 MPa was 107~1010 Pa s, which is about ten orders of magnitudes larger and smaller than that of the ammonia-water mixture in the liquid and solid phases (below a peritectic point, 176 K), respectively. ¿ American Geophysical Union 1994