The XMM-NEWTON satellite is the ESA X-ray spaceborne observatory covering the soft X-ray portion of the electromagnetic spectrum. XMM-NEWTON has been put in orbit on December, 10th 1999 by an Ariane 5 single launch. The spacecraft has a conventional thermal design that takes full advantage of the stable environment provided by its high altitude/long period orbit and by the limited variation of solar attitude angles in order to provide a stable platform for the telescope system. The precise geometry and alignment of the telescope system impose strict temperature requirements so that not only temperature gradients have to be kept small but also, and more importantly, time-variations of the gradients have to be minimised. In the paper, the thermal behaviour of the spacecraft as verified by its thermal test programme is compared with the early in-orbit temperature measurements.