Partial-molar-volume and isentropic-compressibility measurements in water are reported for a systematically chosen series of organic N-bases and their conjugate Brønsted-acid ions so as to investigate the role of the structures of these molecules in their interactions with the water solvent in the ionized and un-ionized states. The results also give the changes of volume and isentropic compressibility upon ionization, hence information on solvent electrostriction due to ionization. The electrostriction effect is related to the entropy of ionization in cases where suitable reliable information exists. The molecular structures are mainly aromatic and saturated heterocycles with one or two N atoms in the ring, or rings with an N and an O atom. Comparisons are also made with a “linear” N-base, ethylenediamine. The results for these various molecules enable the volumetric effects of the following molecular structure changes to be accurately evaluated: effect of one or two N atoms in a ring; effect of CH 2·CH 2 bridging with respect to hydrophobicity; effect of replacement of O by N and by CH 2 and of N by CH, or NH by CH 2 in a ring; introduction of aromaticity; position of N atoms in isomeric heterocyclic structures; and number of CH 2 groups in a ring. The results from this series of comparisons enable some conclusions to be reached regarding additivity of “group contributions” and the role of structural specificity in the volumetric properties of organic bases and their cations in aqueous solution.