Geometrical measurements of pore microstructure indicate that the pore volume distributions for sandstones can be separated into two regimes: a short-length regime with fractal behavior and a long-length regime with no fractal behavior. The fractal part of the distribution is statistically described by a power law, and it appears to be governed by the growth of minerals and cements in the pore space. The nonfractal or Euclidean part of the distribution is dominated by a characteristic length, and it apparently reflects the original unaltered porosity. Two of the sandstones have both fractal and Euclidean regimes, and two of the sandstones have only the fractal regime. The latter two samples are more highly cemented than the former two. Fractal porosities calculated from the fractal parameters measured on fracture surfaces agree with the porosity in the fractal regime measured on thin sections. Criteria developed to determine the relative porosity associated with the fractal and Euclidean regimes could be used to characterize the amount of diagenetic alteration of the pore volume. Photographs are used to support conclusions derived from the geometrical measurements. ¿ American Geophysical Union 1988