A double resonance action spectroscopic method has been applied to measure low-J rotational transitions of H2D+ and D2H+ up to frequencies of 1.5 THz. The uncertainties of two known lines of H2D+ and three known lines of D2H+ have been improved substantially, and the 211←212 H2D+ transition measured for the first time. In addition, we derived spectroscopic parameters for both molecular cations using the Euler formalism. © 2016 Elsevier Inc.