Investigations into the extent of structuring present in phosphonium based ionic liquids (ILs) have been carried out using photochromic molecular probes. Three spiropyran derivatives containing hydroxyl (BSP-1), carboxylic acid (BSP-2) and aliphatic chain (C14H29) (BSP-3) functional groups have been analysed in a range of phosphonium based ionic liquids and their subsequent physico-chemical interactions were reported. It is believed that the functional groups locate the probe molecules into specific regions based upon the interaction of the functional groups with particular and defined regions of the ionic liquid. This structuring results in thermodynamic, kinetic and solvatochromic parameters that are not predictable from classical solvent models. BSP-1 and BSP-2 exhibit generally negative entropies of activation ranging from -50 J K-1 mol-1 to -90 J K-1 mol-1 implying relatively low solvent–solute interactions and possible anion interactions with IL polar functional groups. Higher than expected activation energies of 60 kJ mol-1 to 100 kJ mol-1 obtained for polar probes maybe be due to IL functional groups competing with the charged sites of the merocyanine (MC) isomer thus reducing MC stabilisation effects. Differences in thermal relaxation rate constants (2.5 × 10-3 s-1 in BSP-1 and 3 × 10-4 s-1 in BSP-2 in [P6,6,6,14][dbsa]) imply that while the polar probe systems are primarily located in polar/charged regions, each probe experiences slightly differing polar domains. BSP-3 entropies of activation are positive and between 30 J K-1 mol-1 to 66 J K-1 mol-1. The association of the non-polar functional group is believed to locate the spiropyran moiety in the interfacial polar and non-polar regions. The thermal relaxation of the MC form causes solvent reorientation to accommodate the molecule as it reverts to its closed form. Slow thermal relaxation rate constants were obserevd in contrast to high activation energies (5 × 10-4 s-1 and 111.91 kJ mol-1 respectively, for BSP-3 in [P6,6,6,14][dbsa]). This may be due to steric effects arising from proposed nano-cavity formation by the alkyl chains in phosphonium based ILs.