C o m m e n ts on th e I n te r a c tio n E n e rg y o f a Q u a n tu m O s c illa to r in th e B la c k b o d y R a d ia tio n F ie ld W. Eckhardt Abteilung Mathematische Physik, Universität Ulm Z. Naturforsch. 43a, 314-316 (1988); received January 23, 1988 Recent results on the interaction energy of a quantum oscillator in a radiation field are critically reviewed and compared with the directly calculated thermal expectation values. The proper thermo dynamic definition of the interaction energy is given.