We report on ultrasonic investigations of the frequency dependent and
temperature dependent sound velocity in C60/C70 compounds. Although th
e wellknown phase transitions at about 260K and 155K are clearly seen
in our experiments, concerning the absolute values of the sound veloci
ty our results differ markedly from those found by means of low freque
ncy vibrating reed techniques on C60 single crystals. Reasons for this
unexpected behaviour are discussed briefly.