A collective Thomson scattering diagnostic has recently come into operation at JET. The diagnostic uses a 500kW, 140GHz gyrotron as the source of probing radiation and will be used to measure the spatially resolved velocity distribution of fast alpha particle populations when JET enters the DT phase. First measurements were aimed at the measurement of the thermal ion feature. The diagnostic behaved as expected and the observed signal to noise ratios were satisfactory, indicating that the diagnostic should give the expected performance for observation of fast ion and alpha particle populations in the next operational phase of JET.