We have studied experimentally how the change in solution viscosity affects the convective instability in the acid-catalyzed chlorite oxidation of tetrathionate. The increase in viscosity leads to the decrease of instability; the wavelength of the emerging patterns increases while the fingers evolve slower. We have observed only a minor change in the longtime behavior: the region where a single cell develops slightly increases and the frequency of the occurrence of tip splitting decreases on increasing viscosity. (c) 2005 Elsevier B.V. All rights reserved.