III Zw2 was observed with XMM-Newton in July 2000 .
Its X-ray spectrum can be described by a powerlaw of photon index \Gamma \approx 1.7 with a Gaussian line at 6.7 keV .
There is no significant evidence of intrinsic absorption within the source or of a soft X-ray excess .
Multi-wavelength light curves over a period of 25 years show related variations from the radio to the X-rays .
We interpret the radio to optical emission as synchrotron radiation , self-absorbed in the radio/millimeter region , and the X-rays as mainly due to Compton up-scattering of low-energy photons by the population of high-energy electrons that give rise to the synchrotron radiation .