An ultrasonic method for continuously assessing changes in the shear modulus of hydrating cementitious materials after casting, through setting and early strength gain, is presented. In the test method, reflected shear waves from the interface between the material of the form and the cementitious material are monitored. The test procedure for obtaining the ultrasonic test data and the inversion subroutines for assessing the shear modulus of the cementitious material at different stages of hydration are described. Results from a test program showing the response of ultrasonic signals of 1 MHz frequency reflected from the interface between the form and mortar are presented. A form made of polymethyl-methacrylate (PMMA), was used in the study. The observed experimental trends are explained considering reflection at the interface between two visco-elastic materials. It is shown that shear modulus can be determined immediately after casting and the increase in shear modulus can be sensitively monitored through setting and early strength gain. The shear modulus assessed at 1 MHz exhibits a five orders of magnitude increase in the first 24 hours after casting. The rate of increase in the shear modulus is the most rapid before initial set. The rate of modulus decreases steadily through final set and early strength gain. It is shown that there is a complete phase reversal in the reflected waves with time. The reversal corresponds in time with the final setting time determined using ASTM C 403 and it occurs when the shear modulus of the mortar is almost equal to the shear modulus of PMMA.