The microhardness Hv of xZnO.(50-x)PbO.50B2O3 glasses has been studied as a function of ZnO content and gamma dose. Hv shows a maximum at about 15 mol.% ZnO. 11B NMR suggests the formation of non-bridging oxygen ions in the borate network as ZnO is added in the PbO.B2O3 glass. Both the density and the molar volume decrease with the increase of ZnO. Irradiation with gamma rays lowers the microhardness, whereas heat treatment causes its increase. Infrared investigations indicate that radiation induced defects can activate phase separation processes when the irradiated glass is heat treated.