Two pore domain (2P) family of potassium channels are widely expressed and regulate the excitability of neurons and other excitable cells. These channels have been shown to function as dimers but heteromerization between the various channel subunits has not yet been reported. Here we demonstrate that two members of the TASK subfamily of potassium channels, TASK-1 and TASK-3 can form functional heterodimers when expressed in Xenopus laevis oocytes. To recognize the two TASK channel types, we took advantage of the higher sensitivity of TASK-1 over TASK-3 to physiological pH changes and the discriminating sensitivity of TASK-3 to the cationic dye ruthenium red (RR). These features were clearly observed when the channels were expressed individually. However, when TASK-1 and TASK-3 were expressed together, the resulting current showed intermediate pH sensitivity and RR insensitivity (characteristic of TASK-1), indicating the formation of TASK-1/TASK-3 heterodimers. Expression of a tandem construct in which TASK-3 and TASK-1 were linked together, yielded currents with features very similar to those observed when co-expressing the two channels. The tandem construct also responded to AT1a angiotensin II receptor stimulation with an inhibition that was weaker than the inhibition of homodimeric TASK-1 and greater than that shown by TASK-3. Expression of epitope-tagged channels in mammalian cells showed their primary presence in the plasma membrane consistent with their function in this location. Heteromerization of 2P potassium channels may provide a greater functional diversity and additional means by which they can be regulated in their native tissues.