We study the performance of single- and double- THick Gas Electron Multiplier (THGEM) detectors in pure Hydrogen (H2) and Deuterium (D2) at low pressures, in the range of 100–450 torr. The effect of the pressure on the maximum achievable gain, ion-back flow and long-term gain stability are investigated for single and double cascade detectors. In particular, it was found that maximum achievable gains above 104, from single-photoelectrons avalanche, can be achieved for pressures of 200 torr and above; for lower pressure the gains are limited by avalanche-induced secondary effects to a values of around 103. The results of this work are relevant in the field of avalanche mechanism in low-pressure, low-mass noble gases, in particular for applications of THGEM end-cap readout for active-target Time Projection Chambers (TPC) in the field of nuclear physics and nuclear astrophysics.