In this paper we report the results of surface pressure-area measurements and fluorescence microscopy of Langmuir monolayers of bipolar lipids extracted from the thermophilic archaeobacterium Sulfolobus solfataricus. We have analysed the two-dimensional phase behaviour of the polar lipid extract, some of its subfractions and the hydrolytic fractions glycerol dialkyl glycerol tetraether (GDGT) and glycerol dialkyl nonitol tetraether (GDNT) in the temperature range 25-45-degrees-C. For comparison, films of diphytanoyl phosphatidylcholine were also studied. Only the isotherms of the polar lipid extract exhibit a plateau region. The molecular areas of GDGT and GDNT, even at the collapse pressure, are much larger than those of the other lipid fractions. For the former compounds a U-shaped hydrocarbon chain conformation mixed with an extended chain arrangement is suggested, while all the other fractions display always a vertical arrangement.