The viscoelastic properties of three linear polydimethylsiloxanes (PDMS) of high molecular weights are investigated using rheometrical as well as ultrasonic tests over a large range of temperatures. Classical shear rheometrical measurements are carried out in the low frequency range from 10(-1) to 10(2) rad s(-1) between -50 and + 20 degrees C. The frequency range is enlarged using the time-temperature superposition principle, allowing coverage of about 6-7 decades of pulsation. Ultrasonic tests use an inclined incidence wave reflection technique to measure the complex shear mechanical impedance from 1.5 to 25 MHz, between -10 and -50 degrees C. Rheometrical and ultrasonic experiments are then combined for the three PDMSs at the same reference temperature. They give the reduced shear elastic and loss moduli for reduced frequencies covering 10 decades. A discrete relaxation time spectrum is first deduced from the master curve in each case. More accurate predictions may be obtained using a molecular weight distribution and BSW (Baumgaertel-Schausberger-Winter) model for polydisperse systems.