Photo-crosslinking of unsaturated methacrylic side groups attached to copolymers of polyether urethane in the presence of benzoin methyl ether as photolabile initiator had been investigated. Longitudinal ultrasonic attenuation in the solid form of these materials has been measured at 2, 4, 6, and 8 MHz in the temperature range 240-360 degrees K before and after the photo-crosslinking process and in combination with difunctional acrylate ester monomer. The occurance of crosslinking and the presence of secondary relaxation process beta -relaxation was confirmed. The activation energies, as well as the relaxation frequencies have been obtained. Elastic moduli in these materials were calculated and related to the composition before and after crosslinking as well as to the difunctional acrylate ester monomer.