The lifetimes of positrons have been measured for network polymers based on polyethers. From the temperature dependence of the lifetime of ortho-positronium (o-Ps), tau(3), for the network polymer of poly( ethylene oxide-co-propylene oxide) [P(EO/ PO)], an onset temperature for limited local motions of molecules, T-gamma, and the glass transition temperature, T-g, were determined to be 57 and 201 K, respectively. For the network polymer of poly [EO-co-2-(2-methoxyethoxy) ethyl glycidyl ether] [P(EO/ MEEGE)], T-gamma and T-g were determined to be 57 and 185 K, respectively. For both specimens, above 270 K, the observed linear temperature dependence of tau(3) was attributed to the thermal expansion of open spaces in a liquid state. In the temperature range between T-gamma and 270 K, for the P(EO/MEEGE) network, rs was longer and its intensity was smaller than those for the P(EO/PO) network. These results were attributed to the increase in the size of open spaces for the P(EO/MEEGE) network polymer and the blocking of these regions by motions of side chains and chain ends.