Acid-sensitive background two-pore potassium channels (K_2P) are leak channels that are open at all membrane potentials and are important contributors to the control of cellular excitability. The members of acid-sensitive K_2P channels; K_2P3.1 and K_2P9.1 share 54% homology with each other, and 51% homology with K_2P15.1 (Kim et al., 2000). K_2P3.1, K_2P9.1 and K_2P15.1 are widely expressed, and demonstrate co-expression in an array of tissues including the pancreas, placenta, kidney and lung. K_2P3.1 and K_2P9.1 show functional expression in heterologous expression systems, they are potassium selective and are strongly inhibited by acidic pH. A single nucleotide polymorphism was found at position 95 of the selectivity filter of K_2P15.1, when cloned in 2001 (GYG to EYG), and to date both isoforms have been shown to be non-functional when expressed in COS-7 cells (Kim et al., 2001). Here we investigated the interaction between both K_2P15.1 EYG and K_2P15.1 GYG and other acid-sensitive K_2P channels. We used two-electrode voltage-clamp to measure the functional activity of the acid-sensitive K_2P heterodimers. Xenopus laevis oocytes expressing K_2P3.1 or K_2P9.1 exhibited a pH-dependent current, typical of acid-sensitive K_2P channels (4.6µA (±2.0µA, n=25) and 5.2µA (±3.8µA, n=27) respectively at a membrane voltage potential of 60mV at pH 7.6). A small current (1.2µA ± 1.1µA, n=9) was seen for K_2P15.1 EYG when expressed in oocytes, but K_2P15.1 GYG (0.45µA ± 0.28µA, n=13) was no different from water injected controls at 60mV in pH 7.6 (0.35µA, ± 0.24µA, n=20). Co-expression of K_2P15.1 EYG or K_2P15.1 GYG alongside K_2P3.1 or K_2P9.1 did not alter the current recorded. A K_2P3.1ΔV411 mutant channel tagged with green fluorescent protein (eGFP) was used as a tool to look at the heterodimerisation using immunofluorescence microscopy. The K_2P3.1ΔV411 mutant is unable to bind cytosolic protein 14-3-3, and this loss of interaction has been shown to prevent surface expression of the channel (O’Kelly et al., 2002). Here the mutant K_2P3.1ΔV411 channel was expressed in COS-7 cells, and was able to overcome intracellular retention by co-expressing with wild type K_2P3.1, K_2P9.1 or K_2P15.1. COS-7 cells transfected with the eGFP-tagged K_2P3.1ΔV411 mutant alongside wild type channels were analysed using flow cytometry to quantify the level of surface expression. In addition the interaction between the three acid-sensitive K_2P channels was characterised at a biochemical level using co-immunoprecipitation. Together this data provides the first evidence that K_2P15.1 EYG is functional, and that K_2P15.1 interacts with K_2P3.1 and K_2P9.1. Given that these channels are co-expressed in a number of tissues the physiological significance of these results is far reaching.