Giant magnetoelastic effect compounds GdNi 1−x Co x are investigated using transmission electron microscopy (TEM). Compounds with x=0.3 were previously identified as transitional in this solid solution and found to contain many previously unknown microstructural features which proved difficult to identify via scanning electron microscopy (SEM) techniques. Focused ion beam (FIB) lift-out technique was used to prepare TEM samples of these regions, and high resolution TEM (HRTEM), selected area electron diffraction (SAED) and scanning TEM (STEM) techniques were used in this study to identify the features. The previously seen stitches correspond to mirror twins at least as long as the lift-out specimen (approximately 15µm), and the dendrites radiating from these twins were found to be anti-phase boundary (APB)-like features with very slight variations in lattice parameters. When annealed at 700 o C for 18 days the microstructure was shown to change, eliminating the irregular APBs and forming instead straight regions emanating from the twin in a chevron pattern. This new form of APB was determined to be similar to those found in the as-cast sample only much smaller.