The present work explores the effects of melt chemistry on diffusion controlled creep of partially molten labradorite plagioclase (An 50) at anhydrous conditions. Using sol-gel and hot pressing techniques we produced: 1) nominally melt-free samples (Lab), with < 1 vol. % residual glass confined solely to multiple grain junctions; 2) SilLab1 and SilLab5 partially molten samples containing respectively 1 and 5 vol. % excess amorphous silica, resulting in partial melts wetting numerous grain boundaries as thin (< 10 nm) amorphous films. Energy dispersive X-ray analysis showed that the amorphous phases in Lab, SilLab1 and SilLab5 samples contained about ~ 70, ~ 85 and ~ 95 wt. % Si02, respectively. Infrared spectroscopy showed that the initial traces of water (~ 0.05 wt. %) were dried out by annealing in air above 1100°C. Uniaxial creep tests performed at 1100-1250°C and 3-60 MPa flow stresses showed dominantly linear viscous flow, with a strong grain size dependence indicating grain boundary sliding and diffusion control. Counter-intuitively strength and activation energy increased with the content of melts, but in accord with the silica content of the latter, that is with their polymerization state. Our results show that the kinetics of grain boundary diffusion controlled creep strongly depends on melt chemistry. Instead of acting as shortcut for diffusion, thin films of highly viscous amorphous phases may in turn considerably reduce grain boundary transport properties.