A new "Langmuir" trough is described which allows accurate, symmetric compression and expansion of a spread or adsorbed film of molecules. Compression/expansion of films is equally easy at an air-water (A-W) interface or oil-water (O-W) interface. Several different modes of operation of the trough are described which allow measurement of surface/interfacial pressure (pi)-area per molecule (A) isotherms and measurement of dilatational rheology of interfacial films. Equlibrium and dilatational measurements are consistent with those obtained via other techniques. The pi-A isotherms of spread films of the proteins, beta-lactoglobulin (beta-L) and bovine serum albumin (BSA) are considerably more expanded at the O-W interface compared to the A-W interface.