  We have measured the optical and mechanical loss of commercial silicon
nitride membranes. We find that 50 nm-thick, 1 mm^2 membranes have mechanical Q
> 10^6 at 293 K, and Q > 10^7 at 300 mK, well above what has been observed in
devices with comparable dimensions. The near-IR optical loss at 293 K is less
than 2E-4. This combination of properties make these membranes attractive
candidates for studying quantum effects in optomechanical systems.
