  We present high-resolution, high-sensitivity radio images of the
ultra-luminous infrared galaxy (ULIRG) IRAS 23365+3604. We performed
contemporaneous observations at 1.7 and 5.0 GHz, in three epochs separated by
one year from each other, with the European very long baseline interferometry
Network (EVN). We also present complementary Multi-Element Radio Linked
Interferometry Network (MERLIN) at 1.6 and 5.0 GHz, and archival Very Large
Array (VLA) data, taken at 1.4 and 4.9 GHz. We find that the emission at ~5.0
GHz remains quite compact as seen at different resolutions, whereas at ~1.7
GHz, high resolution imaging reveals some extended structure. The nuclear
region has an approximate linear size of 200 pc and shows the presence of two
main emission components: i) one with a composite spectrum due to ongoing
non-thermal activity (probably due to recently exploded supernovae and AGN
activity), ii) another one with a steep spectrum, likely dominated by an old
population of radio emitters, such as supernova remnants (SNRs). Radiative
losses are important, so re-acceleration or replenishment of new electrons is
necessary. We estimate a magnetic field strength of 18 \mu G at galactic, and
175 \mu G at nuclear scales, which are typical for galaxies in advanced
mergers.
