Densities and speeds of sound were measured for six binary mixtures of water + room temperature ionic liquids (RTILs) of type [C(n)py][X] or [C(n)4mpy][X] (where n = 6 or 8, [X] = Cr- or Br-) at T = (308.15 and 318.15) K at atmospheric pressure. The molar excess volumes (V-m(E)) excess speeds of sound (nu(E)), and excess isentropic compressibilities (kappa(E)(s)) were calculated from the experimental data. Addition of a small amount of RTIL drastically increased the densities of water initially. The V-m(E) values for water + chloride-based RTILs were more negative across the composition compared to the mixtures with bromide-based RTILs; the negative magnitudes, however, decreased as the alkyl chain length and temperature increased. Speeds of sound of the mixtures could be adequately predicted by the collision factor theory and Nomoto equation. The nu(E) values were positive, while kappa(E)(s) values were negative across the composition. The negative V-m(E) and kappa(E)(s) values in general suggest the predominance of strong heterointeractions, which become enhanced with the increase in hydrocarbon chain length. Analysis of partial excess molar volumes or isentropic compressibilities and their standard transfer functions revealed that, compared to those of the Cl- anion, the Br- anion has the weakest water center dot center dot center dot RTIL interactions.