A horizontal co-current stratified two-phase flow was studied in a duct with a rectangular section. Its dimensions were such that the top and side walls did not influence the interfacial waves. The film thickness was almost of the millimeter. The Reynolds number based on the mean height and velocity in each phase varied from 0 to 1.16•105 in air and from 69 to 271 in water. These experimental conditions allowed the observation of the smooth, 2D wavy and 3D wavy stratified flow patterns. The interfacial waves were instantaneously measured on a 100 mm long line segment in the longitudinal midspan plane of the channel with the Level Detection and Recording technique. These measurements yielded statistical characteristics and highlighted the spectral differences between the 2D and 3D wavy stratified flow patterns.