FTIR absorption spectra of water-containing ethene:Ar matrices, with compositions of ethene up to 1:10 ethene:Ar, have been recorded. Systematically increasing the concentration of ethene reveals features in the spectra consistent with the known 1:1 ethene:water complex, which subsequently disappear on further increase in ethene concentration. At high concentrations of ethene, new features are observed at 3669 and 3585 cm(-1), which are red-shifted with respect to matrix-isolated nu(3) and v(1) O-H stretching modes of water and the 1:1 ethene:water complex. These shifts are consistent with a pi(...)H interaction of a 2:1 ethene:water complex of the form ((C2H4H)-H-...-O-(HC2H4)-C-...). The analogous (C2D4)(2)H2O complex shows little shifting from positions associated with (C2H4)(2)H2O, while the (C2H4)(2)D2O isotopomer shows large shifts to 2722.3 and 2617.2 cm(-1), having identical nu(3)(H2O)/nu(3)(D2O) and nu(1)(H2O)/nu(1)(D2O) values when compared with monomeric water isotopomers. Features at 3626.1 and 2666.2 cm(-1) are also observed and are attributed to (C2H4)(2)HDO. DFT calculations at the B3LYP/6-311+G(d,p) level for each isotopomer are presented, and the predicted vibrational frequencies are directly compared with experimental values. The interaction energy for the formation of the 2:1 ethene:water complex from the 1:1 ethene:water complex is also presented.