In this paper we report the results of both an experimental and theoretical study of the halfwidths of two transitions of water vapor. Measurements on the lines of the H 2 16 O and H 2 18 O isotopomers located at 325.1 and 203.4 GHz, respectively, were carried out in the temperature range 300 -393 K, with N 2 and O 2 as perturbing gases. The foreign-broadening coefficients and their temperature-dependence parameters were determined assuming a Voigt profile and the usual temperature dependence for the halfwidth. The retrieved values are compared to values calculated using the complex semiclassical formalism of Robert and Bonamy. The assumed intermolecular potential is a combination of electrostatic and atom-atom components. This last contribution is defined as the sum of pairwise Lennard-Jones 6 -12 interactions between the atoms of H 2 O and the atoms of the perturbing molecules expanded to eighth order. Also calculated are the pressure-induced shifts of the spectral lines for temperatures from 200 to 400 K. Calculated and experimental results are in good agreement, within ฯฎ3.2%, except for the N 2 -broadening temperature coefficients, for which there are discrepancies as high as 23%. Air-broadening parameters are determined following the classical relation: โฅ (air) ฯญ 0.79โฅ (N 2 ) ฯฉ 0.21โฅ (O 2 ).