The structure of the monolayers formed by a group of monodisperse nonyl phenol ethoxylates with parasubstituted 1-butylpentyl chains (para-(C 4 H 9 ) 2 CHC 6 H 4 (OC 2 H 4 ) n OH, abbreviated to BNPE n , n ) 4, 8, and 12) at the air-water interface has been determined by surface tension measurements and neutron reflection. The critical micellar concentration (cmc) was found to be 1.1 ( 0.3 × 10 -5 M for BNPE 4 , 4.0 ( 0.3 × 10 -5 M for BNPE 8 and 8.0 ( 0.3 × 10 -5 M for BNPE 12 and the limiting area per molecule (A cmc ) at the cmc to be 46 ( 3, 61 ( 4, and 75 ( 5 Å 2 , respectively. The values of A cmc are almost identical to those obtained from dodecyl ethoxylates (C 12 E n ) for the same size of the headgroups, suggesting that A cmc for these nonionic surfactants is determined by the size of the headgroups and is not affected by the chemical structure of the hydrophobic chains. The thicknesses of the nonyl phenol layers projected onto the surface normal direction were found to be 18 ( 3 Å for BNPE 4 , 19 ( 3 Å for BNPE 8 , and 22 ( 3 Å for BNPE 12 . In all cases they were about twice as thick as the fully extended chain, suggesting a broad distribution of the hydrophobic chain across the layer and hence a strong mixing of the chain with the ethoxylate groups.