Interfacial tension measurements are employed to explore the spreading behavior of predominantly hydrophobic poly(styrene)-poly(ethylene oxide), PS-PEO, diblock copolymers at air/water and cyclohexane/ water interfaces. Two copolymers viith 7%- and 15.5%-PEO are examined in this study. The former is expected to have a PS block limiting area in air roughly equal to the limiting PEO pancake area, whereas the latter is expected to have a limiting PS block area in air approximately 3 times smaller than the limiting PEO pancake area. At the air/water interface the 7%-PEO copolymer does not spread well, which is attributed to interference from the hydrophobic PS block. In contrast, the 7%-PEO copolymer spreads well at the cyclohexane/water interface,producing an isotherm with a terminating mean molecular area 3 times smaller than that obtained at the air/water interface. The 15.5%-PEO copolymer spreads well at both the air/water and cyclohexane/water interfaces due to less interference from the smaller hydrophobic PS block. These observations are compared to compression isotherms, and the results are discussed in tel ms of the solvating nature of the adjacent cyclohexane phase for the PS block.