We present simultaneous ultraviolet and optical observations of five flares on the very late type M dwarf Gl 866 (dM5.5e). A procedure to estimate the physical parameters of the flaring plasma has been developed using a simplified model of the flare and a comparison of observed and computed Balmer decrements. With this procedure we have determined the optical thickness, electron temperature and electron density of the flaring plasma at flare maximum for three spectroscopically observed flares. The three spectroscopically observed flares cover up to 5 percent of the stellar surface and have areas similar to solar flares.