In this work the wavy interface of stratified oil-water flows was investigated using wire conductance probes. The experiments were carried out in a 38 mm ID acrylic pipe using water and oil (Exxsol D140 oil: rho(o) = 830 kg m(-3), mu(o) = 0.0055 kg m(-1) s(-1)) as test fluids. High-speed imaging revealed that almost two-dimensional interfacial waves develop at the inlet junction for input oil-to-water flow rate ratios different from one. Downstream the inlet section, however, the interface has a complex three dimensional structure with very small amplitude contributions. The structure of such interfaces can be properly investigated from the power spectrum of the conductance probe signal. A rigorous and detailed methodology is presented for estimating the power spectrum of the interface signal that is based on the Wiener-Khinchine theorem and makes extensive use of a Fast Fourier Transform (FFT) algorithm. Interface spectra were studied at two locations, close to the inlet of the test section and at 7 m downstream. The results showed that the waves at the inlet have a unique peak frequency of about 19 Hz and that, at the downstream location, this frequency is still present but has a smaller significance compared to that caused by the mechanical vibrations of the set up. This frequency was independent of the flow rates and could be a characteristic of the pair of the test fluids used rather than of the flow. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).