  We present Infrared Space Observatory mid- and far-infrared spectroscopy of
the merging galaxy NGC 6240, an object presenting many aspects of importance
for the role of star formation and AGN activity in [ultra]luminous infrared
galaxies. The mid-infrared spectrum shows starburst indicators in the form of
low excitation fine-structure line emission and aromatic `PAH' features. A
strong high excitation [OIV] line is observed which most likely originates in
the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely
powerful emission in the pure rotational lines of molecular hydrogen. We argue
that this emission is mainly due to shocks in its turbulent central gas
component and its starburst superwind. The total shock cooling in infrared
emission lines accounts for ~0.6% of the bolometric luminosity, mainly through
rotational H_2 emission and the [OI] 63micron line. We analyse several ways of
estimating the luminosities of the starburst and the AGN in NGC 6240 and
suggest that the contributions to its bolometric luminosity are most likely in
the range 50-75% starburst and 25-50% AGN.
