ABSTRACT We have analysed Chandra/High Energy Transmission Gratings spectra of the X-ray emission line gas in the Seyfert galaxy NGC 4151. The zeroth-order spectral images show extended H- and He-like O and Ne, up to a distance r ∼ 200 pc from the nucleus. Using the first-order spectra, we measure an average line velocity ∼−230 km s−1, suggesting significant outflow of X-ray gas. We generated cloudy photoionization models to fit the first-order spectra; the fit required three distinct emission line components. To estimate the total mass of ionized gas and the mass outflow rates, we applied the model parameters to fit the zeroth-order emission line profiles of Ne ix and Ne x. We determined the total mass of ≈5.4 × 105 M⊙. Assuming the same kinematic profile as that for the O iii gas, derived from our analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra, the peak X-ray mass outflow rate was ≈1.8 M⊙ yr−1, at r ∼ 150 pc. The total mass and mass outflow rates are similar to those determined using O iii, implying that the X-ray gas is a major outflow component. However, unlike the optical outflows, the X-ray outflow rate does not drop off at r > 100 pc, which suggests that it may have a greater impact on the host galaxy.