The XMM (X-ray Multi-Mirror) spacecraft is a space-borne observatory for soft X-ray astronomy. It is developed and built under an ESA contract by an industrial consortium led by Dornier Satellitensysteme GmbH. Due to the large dimensions of the whole spacecraft measuring more than 10 m in height and more than 4 m in diameter, it is split into an upper and a lower module for integration and testing. These modules were separately vacuum tested in the Large Space Simulator at ESTEC, allowing for different spacecraft attitudes with reference to the solar simulator beam.This paper reports on the thermal vacuum test of the XMM lower module (LM), which was conducted in January 1999. The successful completion of this test was a further decisive cornerstone in passing the XMM flight acceptance review in late 1999.