We report measurements of optical loss in GaAs/Al2O3 nonlinear waveguides over the spectral range 1.3-2.1 μm in the infrared using a scattering technique. The optical source was a tunable femtosecond optical parametric oscillator (OPO) generating output pulses with durations of 200-250 fs at a repetition rate of ∼90 MHz and an average power of ∼50 mW. Loss coefficients of ∼1.15-2.55 cm-1, corresponding to losses of ∼5-11 dB/cm were obtained from the measured data. The loss decreases with increasing wavelength due to the Rayleigh scattering contribution.