Isobaric specific heat capacities were measured for (1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4) + acetonitrile (MeCN)) and {1-methyl-3-octylimidazolium tetrafluoroborate (OMIMBF4) + acetonitrile) within the whole range of composition and temperatures from (283.15 to 323.15) K. The excess molar heat capacities were calculated from the experimental results and satisfactorily fitted to Redlich-Kister type polynomials for several selected temperatures. Negative deviations from the additivity of molar heat capacities were observed within the whole composition range of (HMIBMF4 + MeCN) and (OMIMBF4 + MeCN). The results obtained have been interpreted in terms of interactions between ionic liquids and acetonitrile. (C) 2009 Elsevier Ltd. All rights reserved.