The effect of alkyl chain length in cation, temperature and pressure on internal pressure was discussed for aprotic ionic liquids characterized by different hydrogen bonding ability resulting from the structure of the cation (disubstituted imidazolium, trisubstituted imidazolium, pyrrolidinium), and from the structure of the anion (tricyanomethanide, thiocyanate, ethyl sulfate, diethyl phosphate, acetate, bis(trifluoromethylsulfonyl)imide). The internal pressure was calculated by means of the acoustic method from the speed of sound, density and isobaric heat capacity data. The obtained results are compared and analysed along with available literature data. The internal pressure of ionic liquids was compared also with those of alkanes, alkanols, esters and diols. The obtained results show that temperature and pressure dependence of internal pressure for ionic liquids reflects the hydrogen bond capability between cation and anion. (C)2018 Elsevier Ltd.