We investigated traveling fronts of the oxidation of sulfite by iodate and by bromate. Isothermal density changes (Delta rho(c)) and enthalpies were determined, the latter being negative in both cases. The Delta rho(c) is negative in the iodate-sulfite system but positive in the bromate-sulfite system. Double-diffusive convection was expected in the bromate system and was observed. The effect of tube diameter and the orientation of the tube on the wave velocity and wave shape were studied, and a gravitational anisotropy was found in both systems. According to the Pojman-Epstein model, only simple convection should have occurred in the iodate-sulfite system. In fact, curved descending fronts with velocities greater than the ascending ones and "fingering" were observed. Several explanations were considered, including the possibility that the current model must be expanded to explicitly treat the effect of a moving interface on the stability of the fluid.