A study of the perturbation scales growing along a liquid jet interface is presented. Cylindrical gas-oil jets are investigated under different ambient pressures. The jet interface is determined through an image analysis method applied to images of the jet, taken with a very short exposure time. By use of one-dimensional wavelet transform applied to the profile of the interface, several characteristic scales can be obtained. These scales are the wavelength of the perturbation, the magnitude of the perturbation and the curvature scale of the profile. The study of the variation of these scales along the axis of different liquid jets shows noticeable discrepancies to the Rayleigh theory.