A surface tensiometer has been devised which is based on the mathematical solution, as a function of the surface tension of a liquid, of the profile of a cylindrical meniscus formed against a flat plate brought into contact with the free surface of a quiescent pool of the liquid. Only the angles of inclination of the meniscus at two arbitrary points are to be actually measured; the angle at each point is known accurately by observing the angle of reflection of a laser beam vertically irradiated onto the point. No knowledge of the contact angle on the plate is required. The tensiometer is suitable for the study of surface adsorption or of insoluble monolayers just the same as in the Wilhelmy plate method, but it is still superior to the Wilhelmy plate tensiometers and other conventional surface balances in its applicability to the measurements in an atmosphere completely insulated from the laboratory air. An apparatus is demonstrated that has been constructed primarily for the measurements of surface tensions of aqueous lithium bromide (LiBr) solutions with higher alcohol additives, i.e., working fluids of absorption heat pumps, under reduced pressures and elevated temperatures.