The viscous behavior of synthetic silica tubes is studied during collapsing. This technique can be applied directly during the modified chemical vapor deposition (MCVD) of light-guiding preform preparation. By performing several experiments with different tube dimensions, the material viscosity can be measured. In addition, information concerning the thermal behavior of the glass tubes during this process (the difference between the outer and inner temperature depending on the process conditions) can be derived, which is important for process control. This MCVD-inherent temperature difference is caused by radiation loss in wavelength regions in which quartz glass is semitransparent.