  We report 18cm VLBI continuum imaging observations at 5 mas resolution for
UGC 5101, NGC 7469, and Mrk 231, all part of a sample of Luminous Infrared
Galaxies which have been shown to have strong VLBI radio cores. The radio
morphology of these three systems on VLBI scales is AGN-like, with well-defined
ridgelines and high-brightness yet spatially resolved components. The structure
and flux densities of these VLBI components are not consistent with starburst
generated radio supernovae of the type found in Arp 220. On scales of 100pc the
radio continuum in all three objects appears to be dominated by an AGN, not a
starburst. Radio emission on larger scales may well originate in a less compact
circumnuclear star-forming region.
  Confirming and extending VLBI imaging of Mrk 231 by Ulvestad et al. (1999),
our continuum image shows a triple structure, with a core and two lobes,
classifying it as a Compact Symmetric Object (CS0). If the southern (primary)
lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobe
advance speed, $v_a \sim 10^{-4}c$, and an age for the jet/compact source, $<
10^6 yr$. We have also imaged the 1667 MHz OH maser emission in Mrk 231, which
is extended on scales of 50--100 milliarcsec (40--80 pc) and probably coincides
with the inner region of the disk which is seen in CO emission and HI
absorption. Among OH megamasers studied at high sensitivity with mas
resolution, Mrk 231 is unique in the stringent upper limits placed upon the
flux density of compact OH structures of the type found in Arp 220 and other
LIGs. It is possible that the circumnuclear environment of Mrk 231 has been
sufficiently disrupted by the emergent QSO that the cool, dense clouds
necessary for such compact masers no longer exist.
