Charge transfer complexes of κ-BETS2FeX4 (X = Br, Cl) which consist of organic donor molecules(BETS) and magnetic anions(FeX4^-) are fascinating systems to study the interplay between magnetism and (super)conductivity. They form segregate stacked crystal structures that BETS and FeX4^- form different types of two dimensional layers separated with each other to form alternative layered structure. In these systems, π electrons in the BETS molecule layers behave as strongly correlated electron system and localized 3d electrons in FeX4^- (S=5/2) molecule layers behave as low dimensional magnetic system. Due to the magnetic interaction between conducting π electrons and localized 3d electrons, which is called π -d interaction, the ? electron system and the 3d electron system are coupled with each other. As a result, these compounds show various interesting conducting properties such as magnetic-field-induced superconductivity and coexistent state of long range magnetic ordering and superconductivity. In addition, it is also suggested that the influence of the π-d interaction appears in the magnetic properties in the 3d electron system. In order to discuss such novel electronic states of π -d interacting systems from the thermodynamic viewpoint, we performed field-angle-resolved heat capacity measurements. In this article, we show our recent results of thermodynamic studies of π-d interacting BETS salts.