  Global VLBI observations at 5 GHz have been performed to study the source
morphology in 10 compact steep-spectrum (CSS) sources selected from the Peacock
& Wall catalogue with the aim of finding asymmetric structures produced by the
interaction with the ambient medium. The combination of these data and earlier
1.7-GHz observations allows the study of the spectral index distribution across
the source structure and the unambiguous determination of the nature of each
component. In seven sources we detected the core component with a flat or
inverted spectrum. In six sources the radio emission has a two-sided morphology
and comes mainly from steep-spectrum extended structures, like lobes, jets, and
hotspots. Only one source, 0319+121, has a one-sided core-jet structure. In
three out of the six sources with a two-sided structure the flux density
arising from the lobes is asymmetric, and the brightest lobe is the one closest
to the core, suggesting that the jets are expanding in an inhomogeneous ambient
medium which may influence the source growth. The interaction between the jet
and the environment may slow down the source expansion and enhance the
luminosity due to severe radiative losses, likely producing an excess of CSS
radio sources in flux density limited samples. The lobes of the other three
asymmetric sources have a brighter-when-farther behaviour, in agreement with
what is expected by projection and relativistic effects. Simultaneous VLA
observations carried out to investigate the polarization properties of the
targets detected significant polarized emission (~5.5%) only from the quasar
0319+121.
