A comparative monomolecular film study of a series of 1,2-di-O-(alpha-D-glucopyranosyl)-sn-glycerols in which the acyl chains were 16:0, 18:0, 19:iso, and 15:omega-cyclohexyl, was carried out at the air/water interface. Area/temperature isobars were obtained in the 20 degrees through 60 degrees C range with the lipids going from fully condensed to fully expanded. It was found that the area/molecule shift between the expanded and condensed states decreased in the order 16:0 and 18:0 > 19:iso > 15-omega-cyclohexyl, with the expanded state of the omega-cyclohexyl glycolipid being the most condensed. Reduction of the area/molecule shift arose through an expansion of the condensed state and a condensation of the expanded state. Isotherms of the straight-chain glycolipids showed only a small expansion on changing the pH from 5.6 to 2.0, but the omega-cyclohexyl glycolipid showed no effect. All the glycolipids studied showed thermal stability up to the 60 degrees C range, in contrast with previously obtained results on similar phosphatidylcholines, which were unstable above 40 degrees C. These results help explain the greater than 70% abundance of omega-cyclohexyl glycolipids in several strains of Bacillus acidocaldarius, whose natural environment is in hot springs with a pH as low as 3 and a temperature of up to 65 degrees C. It is postulated that the ability to overcome the acidity arises through the insensitivity of the polar headgroup and the highly impermeable bilayer, whereas a glycolipid headgroup appears essential for survival at high temperatures.