Depolarization　in　ferroelectric　materials　limits　their　piezoelectric　applications　and　needs　careful　calibration　for　device　constructions.　With　BaTiO3　and　PbSc0.5Ta0.5O3　as　prototypic　ferroelectrics　we　demonstrate　how　to　apply　Resonant　Piezoelectric　Spectroscopy,　RPS,　to　measure　the　direct　and　the　converse　piezoelectric　effects　as　a　function　of　temperature　to　detect　depolarization　via　the　collapse　of　the　piezoelectric　effect　and　the　anomaly　of　elastic　moduli.　Comparison　of　the　RPS　data　with　in-situ　d33　measurements　on　a　BaTiO3　ceramic　shows　that　the　temperature　evolution　of　the　longitudinal　piezoelectric　coefficient　d33　can　be　determined　with　high　accuracy.　This　makes　RPS　a　complementary　and　convenient　method　for　the　investigation　of　polar　phase　transitions　and　the　thermal　evolution　of　the　piezoelectric　effect.　Finally,　the　observation　of　an　elastic　anomaly　concomitant　to　depolarization　indicates　that　elastic　measurements　are　an　indirect　means　to　measure　the　depolarization　temperature　in　ferroelectrics.　©　2022　Elsevier　B.V.