  We present results of simultaneous dual-frequency (2 GHz and 8 GHz) very long
baseline interferometry (VLBI) observations of 12 active galactic nuclei with
prominent jets. Spectral properties of the jets and evolution of their
brightness temperature are discussed. Measured sizes and brightness
temperatures of VLBI features are found to be consistent with emission from
relativistic shocks dominated by adiabatic losses. Physical scenarios with
different magnetic field orientation in the jets are discussed.
