The chemical composition and strengthening heat treatment, involving air quenching and three step aging, ensure a thermal stability of 1424 alloy sheets as for the fracture toughness and fatigue crack propagation rate characteristics after exposure for up to 3000 hrs at 85degreesC. But these characteristics decrease after exposure for up to 4000 hrs. The study of phase transformation under heating in the temperature range of 85-95degreesC, during 300-4000 hrs shows the increase in both the size and volume fraction of delta'-phase (Al3Li), and additional precipitation of its fine particles, and an appearance of special regions which have fine cellular structure. Fine cellular regions essentially differ from the S-1 (Al2MgLi)-phase. Decrease in fracture toughness characteristics after heating for up to 4000 hrs correlates with additional precipitation of fine delta'-phase particles and intensification of special cellular structures formation along the grain boundaries.