We propose a series of 15 ISO observations to monitor the IR variability of
the blazar 3C279, contemperaneously with other observations at gamma ray, X-ray
and optical wavelengths. ISO observations made in PV phase have shown that the
synchrotron break frequency lies inside the ISO range. Determinaton of the
break frequency is a key diagnostic of synchrotron and synchrotron-self-Compton
models. Our proposed observations would;
-  measure the exact synchrotron break frequency, and search for
flux-dependant variations of that frequency;
- search for possible variations in IR flux and spectral slope;
- search for possible correlated variations between the IR continuum
properties and those at other wavelengths.