Light-emitting devices based on a molecularly thin film of a porphyrin derivative have been fabricated and operated under sinusoidal alternating voltage. The general frequency response of luminance of such devices has been studied and compared with device characteristics under direct current mode. Space charge-assisted electron injection has been found to be operative under ac modes. Transient characteristics of electroluminescence have been studied to explain the frequency response in these devices. Frequency response of luminance further showed that ac light of 400 kHz can be obtained from these molecularly thin devices.