An infrared technique to investigate the transfer processes across the aqueous mass boundary layer is presented. This controlled flux technique (CFT) uses heat as a proxy tracer for gases to allow for fast and local measurements of the gas transfer rate. Three different ways of gas transfer velocity measurements are investigated. The first one, the steady state method is a straightforward way to probe the basic unknowns needed to directly compute the gas transfer velocity k. The other two proposed techniques are model based but also yield a deeper insight into the mechanisms of air-sea gas exchange. The decay method works only in a lab environment while the spatio-temporal temperature fluctuation method is well suited for both lab and ocean measurements. The mathematical foundations of the underlying models are introduced together with their practical application.