Summary: The role of alloy composition, grain structure, precipitate microstructure, and precipitate dislocation interactions on the plastic deformation characteristics and the resulting fracture behavior of two isotropic Al-Li-Cu-X alloys designated AF/C-458 (1.8 w/o Li) and AF/C-489 (2.1 w/o Li) was examined. Inhomogeneous deformation due to strain localization from the shearing of the \(\delta'\) (Al\(_3\)Li), \(\theta'\) (Al\(_2\)Cu), and T\(_1\) (Al\(_2\)CuLi) precipitates lead to fine and coarse planar slip for the AF/C-458 and AF/C-489 alloys, respectively. The intensity of this planar slip was predicted through slip intensity calculations using precipitate density measurements, dislocation particle interactions, and grain boundary misorientation-slip continuity statistics. The slip intensity predictions were corroborated through atomic force microscopy (AFM) measured slip height offsets on the polished surface of single aged and 2% plastically strained tensile samples. Our results suggest that the low ductility of AF/C-489 in comparison to AF/C-458 is primarily due to the much larger slip lengths, i.e. grain size, which increased the strain localization and stress concentrations on grain boundaries, thus promoting low-energy intergranular fracture.