Several rodlike 4,4 "-bis(decyloxy)-p-terphenyl derivatives incorporating nonionic hydrophilic groups in the lateral 2’-position (2-oxa-4,5-dihydroxypentyl, 2,5-dioxa-7,8-dihydroxyoctyl, 2,5,8-trioxa-10,11-dihydroxyundecyl, and 2,5,8,11-tetraoxa-13,14-tetradecyl groups) and 2’-(2-oxa-4,5-dihydroxypentyl)-4,4 "-diundecyl-p-terphenyl form well-ordered thin films when spread at the air-water interface. One observes two sharp breaks in the pressure/area isotherms separated by a large plateau. The first break occurs at an area of ca. 0.90 nm(2)/molecule, an area which corresponds to a side-on arrangement of the terphenyl units at the interface. The plateau corresponds to a first-order phase transition. The surface pressure related to this transition significantly rises with an increasing number of oxyethylene units in the hydrophilic lateral groups. Brewster angle microscopic investigations indicate the formation of fluid domains in this region. In some cases these domains coalesce to a homogenous layer. The surface potential is nearly constant in the region of the plateau, which can be explained by a defined collapse due to the formation of a triple layer consisting of a bilayer on top of the monolayer.