Cementitious materials are transformed from a fluid to a solid state due to a chemical reaction known as hydration. These cementitious materials exhibit a continuous change in the mechanical properties with time; there is a steady increase in the stiffness after setting. An ultrasonic test setup and the data analysis procedure, which provide for continuous monitoring of the hydrating cementitious materials from a very early age, have recently been developed. The test procedure for obtaining the ultrasonic test data from cementitious material at different stages of hydration and the theoretical analysis, which allows interpreting the ultrasonic response in terms of the changes in the acoustic shear impedance of the hydrating cementitious material, are presented in this paper. Experimental test results obtained from mortar mixtures of known composition are presented. It is shown that the initial and final setting times correspond approximately with the occurrence of distinctive features in the ultrasonic response. © 2004 Elsevier Ltd. All rights reserved.