Here we present measurements of the electrical conductivity and density for six ionic liquids (ILs) in aqueous solutions at 25.0 degrees C and atmospheric pressure. The analysed ILs are: two different mixtures of AlBr3 + I -ethyl- 3 -methyl imidazolium bromide (called here EMIM-AlBr3), a mixture of AlBr3 + 1-ethyl pyridinium bromide (EPYR-AlBr3) another mixture of AlCl3 + 1-ethyl-3-methyl imidazolium chloride (EMIM-AICl(3)), Iethtyl-3-methyl imidazolium tetrafluoroborate (EMIM-BF4) and 1-ethyl-3-methyl imidazolium ethylsulfate (EMIM-ES, commercially known as ECOENG (TM) 212). For the EMIM-AlBr3 we have tailored (and then measured) two ILs with two different molar fractions of AlBr3 equal to 0.60 and 0.35, so that one is Lewis acidic and the other one Lewis basic. The density is seen to monotonically decrease when water is added, while the electrical conductivity dramatically increases up about 10 times (with a maximum of 30 times) for selected mixtures of each ionic liquid (IL) plus water. For the majority of systems the peak appears at about 50 wt.%, which corresponds to a molar fraction of IL in the mixture about 0.12. We have extracted the equivalent conductance and we have compared it with aqueous solutions of the precursor salts. Finally, as the data measured do not adapt to current theoretical expressions, we have developed a new theoretical approach based in conventional hole theory. (c) 2006 Elsevier B.V. All rights reserved.