Spreading of carbosilane dendrimers containing trimethylsilyl or hydroxyethyl end groups was investigated at the air/water interface. Our observations suggest that the monodisperse, globular molecules of the carbosilane dendrimer with hydroxyl end groups ordered into layers on the water surface. In contrast to the hydrophobic trimethylsilyl ends, the hydroxyl-functionalized dendrimers formed a monolayer at the air/water interface. Surface pressure versus film area isotherms were collected and showed full reversibility, irrespective of the degree of compression. Three equilibrium states of the OH-terminated dendrimer were identified depending on the molecular area. (i) The monolayer was compressible over a remarkably broad range of molecular areas from 1200 to 650 Angstrom(2). I, the transition region, the osmotic pressure varied with concentration according to the power law pi/k T similar to c(m) with m = 15. In combination with the large monolayer thickness of (1.4 +/- 0.1) nm, the steep power law indicates a globular shape for the adsorbed molecules characterized by dense packing of chain segments. (ii) In the range between 650 and 350 Angstrom(2), the dendrimer underwent a sharp transition, presumably into a bilayer structure. Since the surface pressure was almost invariant during compression, the transition is considered to be a first-order phase transition. (iii) Compression beyond 350 Angstrom(2) occurred at a constant pressure, as is typical for an isotropic liquid film. The spreading behavior of the OH-terminated dendrimer was compared with that of a polydisperse hyperbranched polymer of identical chemical composition. In contrast to the dendrimer, the hyperbranched polymer did not show any transitions and exhibited a spreading behavior similar to that of isotropic liquids.