•Liquid resonances in phononic structures for liquid sensor application.•Semi-transparent phononic crystal reflectors for liquid cavity excitation.•Resonances of cylindrical liquid inclusions constituting the phononic crystal.•Control of conventional petroleum products with phononic crystal sensors. Periodic elastic structures attracted a great attention because of their ability to be designed with artificial acoustic properties. The ability to control the wave propagation and the spectral properties of transmitting or reflecting elastic waves allow to design the sensor structures that are able to exhibit a significant improvement in comparison to currently existing approaches. This contribution is specifically focused on the study of liquid induced resonances of solid-liquid periodic composite arrangements at the frequency range corresponding to the structure bandgap. In this work we bring together several structure designs and study their behavior with specific attention to resonances of liquid constituents. Developed sensor structures are investigated in terms of a frequency spectra variation depending on the speed of sound of a liquid analyte that constitutes the periodic arrangement. From the application side, the work is focused at an in-line analysis of conventional petroleum products and their additives. The sensor measuring circuit is only acoustically coupled to the petroleum under investigation that ensures the safety of proposed approach and allows minimizing the explosion risk in an emergency case. Both, numerical simulations and experimental results clearly disclose the ways and conceptual advantages of the phononic crystal based approach for the specified liquid sensor application.