The polymorphic behaviour of many aliphatic amino acids due to being subject to variable temperature conditions has been extensively explored but their response to high pressure stimulus has not. A comprehensive mapping of pressure and temperature dependence and different phases is necessary for the ability to have full control over the behaviour of these materials. The aim of this research is to provide a methodology that draws on complementary techniques and studies to achieve this control. The investigation has been focused on those aliphatic amino acids comprising a linear hydrocarbon tail. Starting from DL-norleucine with a six-carbon aliphatic chain, the study advances through comparison with aliphatic amino acids that either differ in an atom substitution on the hydrocarbon backbone or in its length. The results show how the molecular structure affects the response to temperature and pressure variation. In all cases, differential scanning calorimetry is firstly applied to gain a qualitative evaluation on the thermodynamics and kinetics of a phase transition. This information is then used to precisely plan the variable temperature single crystal X-ray diffraction experiments and drive subsequent high pressure studies. The high-pressure single crystal and powder X-ray diffraction data, together with the observations under temperature change, are then brought together to plot the relative phase diagrams. The experimental data are then fitted to a specific equation of state. The information gained from the diagrams is augmented by energy calculations carried out using the semiclassical density sums method. This approach enables an evaluation and deeper understanding of the solid-state phase transition behaviour of this family of aliphatic amino acids.