This paper presents the initial results of an investigation of the mechanism that controls the operation of a Rijke type pulse combustor with tangential reactant injection. The study is focused on the role played by reacting vortices, generated in the initial section of the shear layers of the injected reactant flow, upon the driving of the pulsations. Detailed spatial distributions of the reaction rates were obtained by an intensified imaging system. They reveal that reacting, vortex-like, structures are formed near the region where the reactants enter the injection duel when the combustor pressure is near its minimum. Subsequently, these reacting structures grow in size and merge with each other. The merging process is accompanied by a dramatic increase in the reaction intensity that occurs when the combustor pressure reaches its maximum. This satisfies Rayleigh's criterion for driving pulsations by a heat addition process. In contrast, images obtained when the combustor is operated in a steady mode contain no periodic coherent structures.