The objective of this work is the characterization of unsaturated cement paste porosity through the use of ultrasonics. The correlation between ultrasonic velocity and porosity in cement paste material is studied based on both micromechanical modelling and experiments. Experimental measurements of ultrasonic longitudinal and transverse velocities as a function of water to cement ratio were performed on cement paste with and without air-entrained adjuvant. The porosity, the pore geometry and the saturation degree were identified as key parameters that affect the material macroscopic elastic properties as well as the longitudinal and transverse velocities. A micromechanical modeling of unsaturated cement paste was proposed. It allowed considerable understanding of such effects and provided physical interpretations of several relations between velocity, porosity and saturation. The findings of this work should be most appropriate as a foundation for an inversion process and improved cementitious material porosity determination by nondestructive methods.