The paper reports on a systematic study of planar optical waveguides in X-cut LiNbO3fabricated by the proton-exchange technique. The extent of the exchange process was monitored by measuring the infrared absorption peak at 3510 cm−1. It was established that the waveguide profile could be accurately modelled by a step index change with Δn= 0.127 at λ0= 632.8 nm and Δn= 0.096 at λ0= 1150 nm. From the mode effective refractive index measurements, the effective diffusion coefficients for the exchange process were calculated. A value for the activation energy for the proton-exchange process was established. The effect of off-Y-axis propagation on the mode effective indices of the waveguide was studied. A waveguide mode cutoff at 29° from the C-axis was established. The lowest optical losses measured in single-mode planar waveguides were 1.4 dB/cm at λ0= 632.8 nm and 1.3 dB/cm at λ0= 1150 nm for light propagating normal to the C-axis.