The crystal structure of the high-pressure phase of potassium hydrogen carbonate, here termed KHCO3 form III, has been solved using single-crystal X-ray diffraction techniques. It adopts triclinic P1̅ symmetry and is formed on direct compression of the monoclinic ambient-pressure phase (kalicinite, form I) via a first-order phase transition at 3.2 GPa (ΔV/V ~4%). We have also used time-of-flight high-pressure neutron powder-diffraction to determine the structural changes in KDCO3 of the form I and III polymorphs to 8.3 GPa. This study indicates that the transition appears to be in response to pronounced displacements of the K+ cations and the cooperative tilting of the (DCO3−)2 dimers. The O···O hydrogen bond distance shows a monotonic compression over the entire pressure range studied with no obvious discontinuity at the phase transition. However, the O-D···O bond angle appears to exhibit an abrupt ~4° decrease across the I–III phase transition with a concomitant change in its pressure dependence. Birch Murnaghan fits to the equation of state data above and below the transition indicate that form III [B0 = 26.5(2.8) GPa, B′ = 5.2(6)] is marginally less compressible than form I [B0 = 22.7(8) GPa, B′ = 4.1(5)].