Isobaric thermal expansivity alpha(p) as a function of temperature and pressure was determined by means of a calorimetric method for a set of ionic liquids with a common cation within the temperature and pressure intervals (278.15 to 348.15) K and (5 to 50) MPa. The selected common cation was 1-butyl-3-methylimidazolium, whereas the chosen anions were tetrafluoroborate, bis(trifluoromethylsulfonyl)imide, hexafluoroantimonate, trifluoromethanesulfonate, and methylsulfate. The results are compared with available literature data, obtained from density measurements. An unusual, negative dependence of alpha(p) against temperature was found for all liquids over the whole temperature and pressure interval. The results are discussed in terms of the differences in the chemical nature of the anions.