A new method for determining the pore structure of porous materials has been developed. The methodology combines relationships correlating the volume of unfrozen pore solution to electrical conductance, the pore solution freezing temperature to pore radii, and the electrical conductivity of the pore solution to temperature. These relationships are used with measurements of the electrical conductance and temperature to obtain pore size distributions of saturated porous materials subjected to a cycle of capillary freezing and melting;Based on this methodology, a conductometric phase transition porosimeter was constructed. The pore size distribution of porous Vycor glass was measured using this porosimeter. The modal neck and body radii measured by this method are 28 and 55 angstroms, respectively. The modal neck and body radii of the same Vycor sample as determined by mercury intrusion porosimetry are 30 and 85 angstroms and by dilatometric phase transition porosimetry, 27 and 49 angstroms, respectively. Modal neck sizes determined by all three methods are comparable. Modal body sizes determined by the two phase transition methods are also comparable. The modal body size determined by mercury porosimetry is significantly larger and this difference is due to problems associated with mercury porosimetry. These problems are discussed in the dissertation;Two of the most significant advantages the conductometric phase transition porosimeter has over currently available commercial porosimeters are the minimal sample preparation which results in less disturbance of pore structure and the ability to determine pore size distributions of large samples, much larger than other porosimeters can handle, thus minimizing sampling error. To illustrate these advantages, pore size distributions obtained from testing 4-inch diameter by 4-inch high portland cement mortar and concrete cylinders are presented. A comparative pore size distribution as determined by mercury porosimetry is also presented for portland cement mortar;The appendix of this dissertation contains a user's manual for the conductometric phase transition porosimeter. This manual contains instructions for operation of the porosimeter, porosimeter circuitry schematics, computer program descriptions, and computer program listing.