Potential energy curves have been calculated for the ground and excited electronic states of KrH and the cation KrH+ by ab initio configuration interaction calculations using effective core potentials for Kr. Quantum defect functions have been determined from the ab initio potentials of the low-lying Rydberg states of KrH and potential energy curves have been generated for higher n (s,p,d) Rydberg states. The resulting bound-bound transition energies are in excellent agreement with experimental data. The interaction of the 5p B (2)Pi state with the 5s and 5p A (2)Sigma and C (2)Sigma(+) states and their predissociation by X (2)Sigma(+) has been treated by multistate complex scaling calculations for both KrH and KrD. Much larger predissociation widths are obtained in KrH than in KrD, in agreement with experimental observations. (C) 2003 American Institute of Physics.