The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave-tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba0.6Sr0.4TiO3 (0.6-BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., [V ''''(Ti) - 2V(O)(center dot center dot)](x) and [V ''(Ba,Sr) - V-O(center dot center dot)](x). The magnitude of the dielectric constant, epsilon', at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, T-m, because T-m was close to room temperature for the 0.6-BST. The dielectric loss, tan delta, diminished as the epsilon' decreased for 0.98 <= A/B <= 1.05, while the tand was much higher for A/B=0.95 having the greatest A-site vacancy loading. The negatively charged V ''(Ba) and V ''(Sr) were mainly compensated by oxygen vacancies and likely partly compensated by holes, h(center dot), which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%-25% for A/B >= 1.00 despite the reduction of the epsilon', whereas T decreased for A/B<1.00 to ca. 10% for A/B=0.95 having the greatest A- site vacancy loading. The results implied that the [V ''''(Ti) - 2V(O)(center dot center dot)](x) for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the [V ''(Ba,Sr) - V-O(center dot center dot)](x) for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.