The speed of sound in the protic ionic liquids (PILs) N-methyl-2-hydroxyethylammonium butyrate (m2HEAB) and N-methyl-2-hydroxyethylammonium pentanoate (m2HEAP) was measured at atmospheric pressure, and over the range of temperatures T = (293.15 to 343.15) K. The speed of sound and density of aqueous mixtures of the ionic liquid were also determined throughout the entire concentration range, within the (298.15 to 333.15) K temperature range and at atmospheric pressure. The excess molar volume, excess isentropic compressibility, excess speed of sound, apparent molar volume and apparent molar isentropic compressibility were calculated from the experimental density and speed of sound values. Furthermore, all the properties were correlated with selected analytical functions. The apparent molar volume of aqueous PILs was analysed by Pitzer-Simonson theory. The speed of sound of the PILs was predicted with the Wu et al. model and the molar compressibility of the same PILs and their aqueous mixtures were calculated from Wadas model. The results demonstrate that the molar compressibility calculated from Wadas model is almost a linear function of mole fraction and can be considered as temperature independent for a fixed mole fraction over the whole composition range. The results were analysed and discussed from the structural changes point of view in aqueous medium. (C) 2016 Elsevier Ltd. All rights reserved.