Cold molecule spectroscopy for constraining the evolution of the fine structure constant

We report precise measurements of ground-state, Lambda-doublet microwave transitions in the hydroxyl radical molecule (OH). Utilizing slow, cold molecules produced by a Stark decelerator we have improved over the precision of the previous best measurement 25-fold for the F-'=2 -> F=2 transition, yielding (1 667 358 996 +/- 4) Hz, and by tenfold for the F-'=1 -> F=1 transition, yielding (1 665 401 803 +/- 12) Hz. Comparing these laboratory frequencies to those from OH megamasers in interstellar space will allow a sensitivity of 1 ppm for Delta alpha/alpha over similar to 10(10) yr.