The method presented herein is the ultrasonic non-destructive testing method. This research work investigated the relationship between ultrasonic measurements and the composition, behaviour and properties of cementitious materials. The instrument used was the Portable Ultrasonic Non-Destructive Digital Indicating Tester (PUNDIT). The longitudinal pulse velocity is the parameter that was measured. The cementitious materials tested were ordinary portland cement and various cement blends. The mineral admixtures used in the blends were fly ash, limestone, ground granulated blast furnace slag and metakaolin. The results of the investigation show that the ultrasonic pulse velocity measured in each case is related to the composition, properties and behaviour of the cement pastes. The results also indicated that there is some level of correlation between the pulse velocity and the compressive strengths of the specimens tested. The findings of this work have considerable importance with respect to in situ testing of concrete structures and mortars.