The weak-field dielectric properties of ordered, disordered, and lead-deficient disordered Pb(Sc0.5Ta0.5)O-3 (PST) ceramics (PST-O, PST-D, and PST-DV, respectively) were investigated under various de-bias conditions. The de-bias field influences the paraelectric-to-ferroelectric transition in PST-O and the relaxor-to-normal-ferroelectric transition in disordered PST-D, Phase diagrams, with respect to de bias and temperature, were proposed for ordered and disordered PST materials. A field-induced ferroelectric state that originated from the residual non-ordered regions in PST-O was revealed by analyzing the Curie-Weiss behavior of the materials under bias. It is shown that, in PST-DV, a de bias cannot induce an abrupt ferroelectric-to-relaxor transition. A dielectric anomaly observed at low temperatures in all three types of PST materials is attributed to a poling process.