Sequential combustion system of GT24 and GT26 engines from Alstom consists of a primary combustor, followed by a turbine and a re-heat combustor. This technology is capable of achieving the most rigorous emission targets at base-load operation, with an additional benefit of delivering very low NOx and CO emissions down to extremely low part load. This paper describes further development of the reheat technology with the help of multiple tools, with special consideration given to atmospheric tests. Prior to more demanding high-pressure combustion tests, full-scale atmospheric tests are utilized in order to down-select the most promising variants in a relatively cost-effective setup. Atmospheric tests are used additionally to gain insight into fundamental physics of reheat combustion, thanks to optical access and applicability of complicated measurement methods. A number of parameters, including NOx, CO, UHC emissions, temperature distribution, flashback margin, pressure drop and — to some extend — combustion dynamics can be assessed with the help of atmospheric tests. In order to increase the applicability of atmospheric test results to high-pressure conditions, the ignition and reaction characteristics of high-pressure flames must be reproduced at atmospheric conditions. This can be done through adjustment of operational parameters such as incoming temperature, as well as using fuel blends with modified ignition properties. The scaling methods as well as recent analyses on improvement areas for this unique combustion technology are discussed.