<p>The testing and optimization of parameters for the direct measurement of specific heat capacity and thermal diffusivity of organic substances in the range of temperature 5–90 °C were investigated. High density polyethylene (HDPE) was selected as widely studied thermoplastic polymer due to its well-known thermophysical properties available in literature. Eicosane (C20), an oligomer selected as a pure substance, and a fatty acid mixture (SA) representing a commercial phase change material (PCM) were also investigated due to the fundamental importance of characterizing the thermophysical properties of PCMs to model their thermal behavior and implement their use in practical applications. After a preliminary analysis on the influence of scanning rate on the PCM phase transition via Differential Scanning Calorimetry (DSC), identifying 1 °C min⁻¹ as the optimal rate to balance thermal inertia and enthalpy accuracy, we determined the best parameters for specific heat capacity determined by comparing the sapphire method and the TOPEM^® technique. Subsequently we compared the thermal diffusivity determined using Laser Flash Analysis (LFA) and the Hot Disk (transient plane heat source) method, evidencing that specific geometric arrangements, such as specimen thickness, are critical for ensuring result convergence. The study highlights the inverse temperature dependence of specific heat capacity and thermal diffusivity, confirming the phase transition as a region of thermophysical indetermination where property values lack physical significance. We propose a critical discussion regarding different strategies and parameters, stressing that cross-technique validation is essential to overcome methodological concerns and achieve the high degree of accuracy required for reliable predictive modeling in thermal energy storage.</p>