Thermosalient materials, or more colloquially called jumping crystals, are a class of materials that exhibit mechanical motion as a response to the thermal stimulus. Such materials, which are capable of extremely fast and, more important, controllable energy conversion are extremely important candidates for active actuating elements, such as flexible electronic switches, displays, artificial muscles, microfluidic valves and gates, heat sensors etc. New examples of thermosalient materials are emerging on a daily basis but yet, due to the exclusivity and individuality of the phenomenon, plus the complexity of analytical methods for its characterization, the reasons behind this colossal self-actuation remain unexplained and this thesis aims to address this lack of understanding. In this thesis two thermosalient systems, N'-2-propylidene-4-hydroxybenzohydrazide and oxitropium bromide, as well as their derivatives were extensively studied. In the case of N'-2-propylidene-4-hydroxybenzohydrazide, existence of the negative linear compressibility was experimentally proven and as well as its coupling to the negative thermal expansion. Raman measurements proved the previous theoretical suggestion of the phonon softening while approaching phase transition, and this softening helps the system transform from one thermosalient phase to another. Also, the mechanism of negative thermal expansion – the main reason for the existence of the thermosalient effect - and negative linear compressibility was fully determined. Oxitropium bromide was extensively studied in this thesis regarding its thermal behaviour prior to the polymorphic phase transition, once again proving the need of the negative thermal expansion for thermosalient properties. Two of its derivatives were prepared: scopolamine bromide hydrate and scopolamine bromide which exhibited thermosalient effect based on two novel mechanisms, previously not reported in the literature. Jumping of the crystals of scopolamine bromide hydrate is caused by the release of the water from the crystal lattice, whereas the crystals of the scopolamine bromide jump without any phase transition or chemical change and its mechanism is not yet understood.