  We report on the results of a two epoch study of the low power Compact
Symmetric Object 4C 31.04. Observations performed with the VLBA at 5 GHz in
1995 and 2000 have yielded images of this source at milliarcsecond angular
resolution. A central core is detected, with bright compact hot spots and
extended lobes on both sides. Model-fitting and other analysis of the data
(brightness profile, difference map) clearly indicate that the source is
expanding. We estimate the velocity of this expansion to be (0.085 +/- 0.016)
mas/yr, i.e. (0.33 +/- 0.06) c in both hot spots. Assuming a constant expansion
velocity, we estimate the kinematic age of the source at 550 yrs. We also study
the spectral index using VLBA observations at 1.3 GHz and MERLIN at 22 GHz. The
derived spectral age is 3000-5000 years in equipartition conditions. The two
estimates are discussed and found to be in agreement, given present
uncertainties.
