Tensile-creep experiments were conducted in the temperature range 100-200 degrees C and stress range 20-83 MPa for a series of magnesium-zinc-yttrium (Mg-Zn-Y) and mangnesium-zinc-zirconium (Mg-Zn-Zr) alloys ranging from 0 to 5.4 wt% Zn, 0 to 3 wt% Y, and 0 to 0.6 wt.% Zr. The greatest tensile-creep resistance was exhibited by an Mg-4.1Zn-0.2Y alloy. The room-temperature yield strength increased with increasing Y content for Mg-1.6-2.0Zn alloys. The greatest tensile strength and elongation was exhibited by Mg-5.4Zn-0.6Zr. This alloy also exhibited the finest grain size and the poorest creep resistance. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress. For applied stresses greater than 40 MPa, the creep exponents were between 4 and 8. For applied stresses less than 40 MPa, the creep exponent was 2.2. The calculated activation energies (Qapp) were dependent on temperature where the Q (app) values between 100 and 150 degrees C (65 kJ/mol) were half those between 150 and 200 degrees C for the same applied stress value (30 MPa). Deformation observations indicated that the grain boundaries were susceptible to cracking in both tension and tension-creep, where at low applied stresses grain boundary sliding was suggested where strain accommodation occurred through grain boundary cracking. Thus grain size and grain boundaries appeared to be important microstructural parameters affecting the mechanical behavior. Microstructural effects on the tensile properties and creep behavior are discussed in comparison to other Mg-based alloy systems.