We present the results of VLBA observations at 6 and 3.6cm for eighteen Compact Steep Spectrum radio sources (CSS) from the B3-VLA CSS sample. In most cases these VLBA images display a "Double/Triple" morphology, and the core is unambiguously detected in seven sources. Multifrequency images allow us to study the spectral properties of the individual source components and to constrain the spectral age. In lobe-dominated sources the radiative ages deduced from the synchrotron theory are in the range of up 5x10^3^ years, if equipartition magnetic field is assumed. Polarized emission is detected at a few percent level for two sources only. They were the only two sources displaying polarized emission in VLA A configuration data, and this implies that beam depolarization is not effective in reducing the integrated fractional polarization for these sources.