One of the leading causes of death in the western world today is heart diseases, and atherosclerotic plaque that ruptures in the coronary arteries is believed to be one of the main causes of heart attacks.The Capacitive Micromachined Transducer has good acoustic matching with fluids, and is considered a good candidate for improving intravascular ultrasound diagnostics.Further evaluation and characterization of a CMUT operating in immersion has been the objective of this thesis, and it is a continuation of the work started in the project Characterization of Capacitive Ultrasound Transducers. The same CMUT have been studied in both, but a backing layer was added to it for the testing done during the work on this thesis.Pulse-echo measurements with the CMUT immersed in rapeseed oil have been performed, and there has also been conducted measurements to find the attenuation of the oil. The results from the attenuation measurements have then been employed to simplify the analysis of the pulse-echo measurements.Any definitive characteristics for the CMUT was not found during these tests due to disturbances in the frequency responses. This is suspected to be because of line resistances and parasitic capacitances in the system. This suspicion is supported by the findings from studying the impedance response of the system.The backing layer added to the CMUT have proved to be very effective in dampening the ringing effect on the pulses emitted by the CMUT. There have also been signs of unexpected alteration of the CMUTs frequency characteristics. One possible source of this is that the process of adding the backing layer have resulted in that the layer exerts force on the CMUT, and by this changing the tension of the membranes.