A working subsolidus phase diagram for the system BaTiO3-Ba5Nb4O15 has been determined by firing sol-gel-synthesized samples over a range of temperatures. The main difference from previous diagrams is the greater extent of the Nb-doped BaTiO3 cubic solid solutions, BaTi1-5xNb4xO3, at lower temperatures with x extending to 0.09 at 900 degrees C, but only 0.05 at 1400 degrees C. Electrical property measurements show that compositions with large x (x >= 0.0025) are highly insulating for pellets sintered at 1300 degrees C in air, followed by a slow cool. Compositions with low x, however, exhibit a residual semiconducting grain core and are not fully reoxidized readily. Composition dependence of the dielectric properties shows a continuous and smooth transition from classic ferroelectric behavior with pure BaTiO3 to normal dielectric response with a temperature-independent relative permittivity of approximately 22-24 for x >similar to 0.08. At intermediate compositions, ranges of both relaxor ferroelectric and quasi-ferroelectric behavior are observed. Possible reasons for an observed anomalous increase in value of the permittivity at the ferroelectric transition temperature at low x, which is superposed on an overall decrease in permittivity with increasing x, are discussed.