Measurements of the electromagnetic (EM) bias in altimeter sea surface ranges at nadir and off-nadir incidence angles were made during the Brigham Young University (BYU) Off-Nadir Experiment (Y-ONE) in the months of March and April, 2003. Tower-mounted C- and Ku-band radars were deployed along with laser rangefinders and a weather station to measure sea surface profiles and environmental parameters. The incidence angle range was nadir to 17¿. An off-nadir bias model incorporating the effects of hydrodynamic modulation of short waves and tilt modulation of long waves was also developed. Both the experimental data set and the theoretical model led to a bias of decreasing magnitude as the incidence angle moves away from nadir. The average measured EM bias at C-band was approximately zero at 14¿ incidence, and a small positive mean bias (1% SWH) was observed at 17¿. By extrapolating from the Ku-band measurements, the mean bias vanished at 18.5¿ for the higher frequency.