The resonator mode frequencies and the interline and intraline tuning behavior of CO2waveguide lasers should depend strongly on precise guide dimension. We describe a compact grating-tuned RF-excited device with a rectangular Al/Al2O3waveguide whose width can be varied during operation. It yields ≃ 50 mW with a continuous tuning range of ≃ 500 MHz on stronger lines; more interestingly, its line selectivity does seem to depend on guide width, as we predict. In particular, the losses of unwanted lines (and thus the tuning range within a desired line) are very sensitive to waveguide width, because the resonator mode associated with an unwanted line may have significant higher order waveguide mode content. This suggests that variable-width waveguides offer one solution to the problem of line-hopping, without prejudice to the power or transverse mode quality obtainable on the desired line.