High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187A, were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This data was used in conjunction with existing high-resolution Hubble Space Telescope STIS observations to evaluate the total HI, DI, OI and NI column densities along the line of sight. Previous determinations of N(DI) based upon GHRS and STIS observations were controversial due to the saturated strength of the DI Lyman-alpha line. In the present analysis the column density of DI has been measured using only the unsaturated Lyman-beta and Lyman-gamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are: log N(DI) = 13.40 +/-0.07, log N(OI) = 14.86 +/-0.07, and log N(NI) = 13.87 +/-0.07 quoted with 2-sigma uncertainties. The measurement of the HI column density by profile fitting of the Lyman-alpha line has been found to be unsecure. If additional weak hot interstellar components are added to the three detected clouds along the line of sight, the HI column density can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Lyman-alpha are excellent. The new estimate of N(HI) toward G191-B2B reads: log N(HI) = 18.18 +/-0.18 (2-sigma uncertainty), so that the average (D/H) ratio on the line of sight is: (D/H) = 1.66 (+0.9/-0.6) *10^-5 (2-sigma uncertainty).
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