An ultrasonic technique for measuring reflection of horizontally polarized shear (SH) waves at different angles of incidence from early-age hydrating mortar and cement paste is described. Measurements from mortar with varying sand content are presented. A poroelastic formulation for predicting wave reflection from hydrating mortar, which considers mortar as a two-phase, water-filled solid skeleton, is developed. In this representation, the solid phase in early-age hydrating mortar consists of sand, products of hydration and unhydrated cement. Reflection at the mortar interface is predicted using the poroelastic theory. Changes in the reflected amplitude of SH waves are used to obtain changes in the water-filled pore space and the shear modulus of the porous skeleton within the two-phase poroelastic idealization. The water-filled pore space in mortar is identified with the porosity in the two-phase poroelastic material. The porosity of mortar obtained from the poroelastic idealization when referring to the volume of cement paste within the mortar is comparable to the results obtained by directly applying the two-phase idealization to cement paste with identical water-to-cement ratio. The reflection of SH waves is strongly influenced by the absolute porosity of the material at any given time. A methodology for real-time, nondestructive monitoring of evolution of porosity in hydrating cement paste and mortar is established. © 2016 American Society of Civil Engineers.