The present study is focused on four isomeric and quasi-isomeric ionic liquids, namely 1-alkyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imides, where alkyl stands for pentyl, isopentyl, 3-pentyl, or cyclopentyl. This series of ionic liquids has formerly been studied as to their densities and transport properties and has shown interesting behaviour with respect to their chemical structure [1]. This work extends our knowledge of the thermal properties of these ionic liquids. The isobaric heat capacity and the thermal stability were measured and put into relationships with the different chemical structures studied. Because of recycling of ionic liquids used, density was measured as a benchmark of the quality of material. Newly, the density data from this work were corrected for their viscosity using experimental viscosity data. Together with measurements of velocity of sound, these data provided basis for calculation of coefficients of isobaric thermal expansivity and isentropic compressibility, quantities that are related to heat capacity. Similarly to previous work, the isomerization was found to have a pronounced effect on studied properties and the present fundamental physico-chemical data are therefore useful both in the design of new ionic liquids as well in theoretical studies and molecular modeling. The data analysis and correlation were carried out using robust non-statistical methods based on mathematical gnostics.