The line tension lambda of the liquid expanded (LE)/gas (G)-phase boundary of a methyl octadecanoate Langmuir monolayer at the air/water interface is measured using fluorescence microscopy combined with optical tweezers. Silica spheres, immersed into the monolayer and manipulated by the tweezers, deform the phase boundary. After switching off the tweezers, the relaxation of the deformed region is dominated by the competition between line tension and hydrodynamic resistance while dipole-dipole forces between the molecules can be neglected. A linear relation between the deformation length and time is found, from which a line tension of lambda = 7.5 pN is deduced. The linearity gives an upper bound for the surface potential differences of both phases. It is shown that viscous forces from the two-dimensional LE surroundings dominate the subphase friction. The optical tweezers enable one to observe relaxations on a shorter time scale, extending the range of measurement of previous techniques toward higher line tension or lower viscosities of the monolayer and of the subphase. (C) 2000 American Institute of Physics. [S0021-9606(00)72013-7].