We investigated the phase stability of the VNbMoTaW and TiZrNbHfTa quinaries and their equiatomic quaternary, ternary, and binary subsets after long-term anneals at 1200-800 °C for 3-300 days. The propensity for forming single-phase solid solutions increased with increasing annealing temperature (vibrational entropy) but there was no correlation with increasing configurational entropy. There was good correlation between the number of BCC constituent elements and single BCC solid solutions, consistent with the Hume-Rothery crystal-structure criterion for extensive solid solubility. We also investigated the mechanical behavior of several pseudo-binary subsystems of the Ti-Zr-Hf-Nb-Ta alloy in which pairs of elements were varied while the concentrations of the remaining three constituent elements were kept constant. Only compositions resulting in singe-phase BCC solid solutions at room temperature were considered. The roles of shear modulus, melting point, and volume misfit on their room-temperature tensile properties (strength and ductility) will be discussed.