Complex periodic and aperiodic behaviors are reported in an unstirred Belousov-Zhabotinsky oscillatory reaction containing different concentrations of polyethylene-glycol. A route from chaos following an inverse Ruelle-Takens-New-house (RTN) scenario is identified. Thus, hydrodynamic effects, being mainly convective, seem to be responsible for the observed RTN scenario. We demonstrate that the medium viscosity, namely the polyethylene-glycol concentration, is a bifurcation parameter for the sequence chaos --> quasi-periodicity --> period-1.