The size of pores in a number of solid samples has been measured by nitrogen adsorption, oxygen adsorption, mercury porosimetry, scanning electron microscopy, and optical microscopy. All the pores in each of the solid samples investigated were straight, had a circular cross‐section, and had similar dimensions. By assuming the existence of contact angle hysteresis, agreement between the microscopy pore radii and mercury porosinietry radii measured both from penetration and from retraction was obtained. Dimensional changes in the solid may have contributed to the pore radii determined by nitrogen adsorption and oxygen adsorption being larger than the radii determined by microscopy.