We study a novel kind of coupling in chemo-hydrodynamic pattern formation driven by a neutralization reaction along a plane interface separating two immiscible liquid phases. The neutralization reaction, during which a surface-active carboxylic acid is converted into a surface-active salt, gives rise to numerous cycles of relaxation oscillations between a fast cellular Marangoni convection with parallel-acting density plumes and a slow finger convection. By means of particle image velocimetry the dynamics of the sub-structured Marangoni cells are characterized while their geometrical aspects are analysed using shadowgraphy. Based on concentration-dependent density measurements and experiments with miscible solutions, the finger convection could be clearly identified as a double-diffusive phenomenon. Furthermore, the interaction of the sub-structured Marangoni cells with the density effects is examined. (C) 2011 Elsevier Ltd. All rights reserved.