A film balance for the investigation of insoluble monolayers at the macroscopic oil–water interface is described. The design of the balance enables simultaneous surface tension and optical reflection measurements under well defined conditions. The oil-phase is contained within a quartz housing whose dimensions are optimized for imaging purposes. The performance of the apparatus is illustrated by the &pgr;,A-isotherm of a phospholipid and ellipsometric images of the morphology of the phospholipid dipolmitoyl-phosphatidyl-ethanolamine at the hexadecane–water interface. ©1997 American Institute of Physics.