Interactions of 1-butyl-1-methylpyrrolidinium tetrafluoroborate (BMPYBF4) and 1-butyl-1-methylpiperidinium tetrafluoroborate (BMPIBF4) with water and acetonitrile (AN) were studied in a comparative manner through the measurements of density and speed of sound at 288.15, 293.15, 298.15, 303.15 and 308.15 K. The measured densities and speeds of sound were used in calculation of apparent molar volumes and apparent molar isentropic compressions of the two ionic liquids (ILs) in water and in AN. Infinite dilution values of the two apparent molar quantities were obtained by extrapolation. Then, these properties were evaluated in terms of IL-water and IL-AN interactions. The effect of presence of an additional cyclic -CH2- group in the structure of BMPIBF4, compared to that of BMPYBF4, on the volumetric and acoustic properties has been examined both in water and in AN. The effect of solvent, H2O or AN, on the solution behaviors of the two ILs is discussed, based on transfer apparent molar volumes and transfer apparent molar isentropic compressions from H2O to AN. Apparent molar isobaric expansions at infinite dilutions were determined. Temperature effects on solution behaviors of the two ILs in H2O and in AN are discussed based on variation of apparent molar properties at infinite dilution with temperature.