Immiscibility was investigated in Group IV – Group V transition metal diboride pseudobinary systems (TiB2 – NbB2, TiB2 – TaB2, ZrB2 – NbB2, ZrB2 – TaB2, HfB2 – NbB2, and HfB2 – TaB2), revealing classic eutectic lamellar microstructures. Autocorrelation analysis of the microstructure length scales was used to validate the eutectic nature of these microstructures. These results indicate that substantial revisions are required for the pseudobinary phase diagrams of Group IV – Group V transition metal diborides available in the literature and explain several discrepancies between available thermochemical literature and reported microstructure results in these systems.