We report experimental and simulation results of capacitance of quantum well infrared photodetectors (QWIPs). We found that the QWIP capacitance displays unusual behavior as a function of voltage and frequency, deviatingfarfrom the constant geometric capacitance value. At high voltages, capacitance starts with a negative value at low frequencies, increases above zero with frequency, and eventually decays to the geometric capacitance value. The magnitude of negative capacitance exceeds the geometric capacitance by more than two orders of magnitude. Negative capacitance arises when the transient current in response to a voltage step is nonmonotonic with time. Simulation shows that this effect is due to nonequilibrium transient electron injection from the emitter resulting from the properties of the injection barrier and inertia of the QW recharging processes. ©1997 American Institute of Physics.