The pore volume and surface distributions of 25 hardened pastes of a hydrated portland cement were determined by water vapor adsorption isotherms. The water to solid weight ratio, w s , used in the preparation of 21 pastes was 0.2, and of 4 pastes it was 0.3. The w s ratio determines the porosity of the paste, and such pastes as are discussed in the present paper are called low-porosity pastes. In the analyses, the two new methods developed by Brunauer and his coworkers were used: the “MP-method” for the micropores, and the “corrected modelless” method for the wider pores. Of the 25 pastes, 8 were previously analyzed by nitrogen adsorption isotherms, using the same methods. Comparison of the water vapor and nitrogen results showed that nitrogen could not penetrate into the micropores, and it could penetrate only into a small fraction of the wider pores. This indicates the presence in low-porosity pastes of very many “ink-bottle” pores, i.e., pores having wide bodies and narrow entrances. The analyses were based on the adsorption isotherms. Desorption equilibration takes very long times, and the indication from the isotherms, as well as from earlier work, was that, given adequate time, very small hysteresis loops would be obtained, or none at all. The pore structure results based on the adsorption branch satisfied the two criteria of correct analysis very well: the cumulative pore surface agreed with the BET surface, and the cumulative pore volume agreed with the uptake of water at the saturation pressure. This is what one would expect for adsorbents which contain predominantly ink-bottle pores. The results showed that with progressing hydration micropores were formed at the expense of wider pores. From one-half to three-fourths of the surface area was located in micropores. The pore volume distribution curves showed that the temperature of hydration had little or no effect on the pore structure. The pastes prepared with w s = 0.3 had a larger pore volume in each pore group than those made with w s = 0.2 , but the distribution of the volume in pores of different sizes was similar for the two w s ratios. The compressive strengths of the pastes were found to depend not only on the degree of hydration and total porosity but also on another factor, which may be the pore volume distribution. The micropores appeared to have a greater effect on the reduction of strength than the wide pores.