Alstoms GT24 and GT26 engines feature a unique sequential combustion system [1, 2]. This system consists of a premixed combustor (called EV), which is followed by a high pressure turbine, a reheat combustor (called SEV) and a low pressure turbine (Figure 1). Recently improvements in NO_x performance of the SEV have been demonstrated. Starting with relatively simple methods numerous design variants have been tested and down selected. Further down-selection has been done with methods of increased complexity. Overall a fast and cost effective development process has been assured. During the development process the variation coefficient and unmixedness measured and calculated for mixing only systems (CFD and water channel) has proven to be a reliable indicators for low NO_x emissions for the real combustion system on atmospheric and high pressure test rigs. To demonstrate this a comparison of both quantities against NO_x emissions is shown. The paper focuses on the NO_x results achieved during this development and its relation to mixing quantities. Using this relation, together with a detailed understanding of the flow characteristic in the SEV burner, reductions in NO_x emissions for GT24 and GT26 SEV burner and lance hardware can be reached using relatively simple methods.