This paper describes selected thermophysical properties of the newly obtained paramagnetic inky, opaque ionic liquid, 1-butyl-3-methylimidazolium tetraisothiocyanato-cobaltate(II) ((BMIm)2[Co(NCS)4]). Some properties of the homologous ionic liquid (IL), 1-ethyl-3-methylimidazolium tetraisothiocyanato-cobaltate(II) ((EMIm)2[Co(NCS)4]), have been presented earlier and are discussed here together with those for (BMIm)2[(Co(NCS)4]. On the basis of experimental densities, kinematic viscosities, conductivities, and surface tensions, some additional parameters have been determined, i.e., expansivity coefficients and surface thermodynamic properties. The influence of structural variation in the 1-alkyl-3-methylimidazolium cation is also discussed. [ABSTRACT FROM AUTHOR]