The industrial applications of Confined Impinging Jets (CIJ) mixers typically involve the mixing of two dissimilar fluids. Previous fundamental studies on the flow dynamics in CIJ mixers have only reported mixing fluids with a viscosity ratio up to 2 in mixers with similar nozzle diameters. In this paper, the onset of a self-sustainable chaotic flow regime was visualised for mixing of dissimilar fluids with a viscosity ratio range from 2 to 9 and for chambers with different injector diameters. Experimental and numerical results show that CIJ mixing chambers are able to efficiently mix fluids with different viscosities. The two sufficient conditions to effectively mix in a chaotic flow regime are the operation above the critical Reynolds number defined by the more viscous fluid and the balance of jets that makes them imping at the mixing chamber axis.