Oil companies have a real need to make a correct assessment of cementing jobs in their we[is. Is the seal ensured between different zones? To do this, sonic and ultrasonic logging tools have been developed. Up to now, the quality of the casing-cement ＇＇contact＇＇ has been analyzed quantitatively, and that of the cement-formation ＇＇contact＇＇ has been analyzed qualitatively by specialists (VDL tool). The greatest progress that could be made with cementing logs was to detect defects in the cement-formation interface. This is what we have done within the framework of a project financed by ARTEP (Association de Recherche sur les Techniques d＇Exploitation du Petrole, made up of Total, Gaz de France (GDF), Institut Francais du Petrole (IFP) and Elf Aquitaine Production (EAP)). Laboratory experiments performed in the Fluid Analysis Service at Boussens involved the injecting of cement between a casing and a simulated formation, with or without the presence of mud having varying thicknesses. Fast or slow formations as well as fast or slow cements were used. The ultrasonic echoes obtained by a ceramic CET sonde were recorded and analyzed. Both theory and the experiments showed that ultrasonic echoes are modified in the presence of mud and/or gas. The relations between the shape of the ultrasonic wave and presence of mud and gas between the cement and the formation were determined. A processing procedure is proposed with its limitations.