A system for reading from and for writing data into electronic tags which may be associated with objects which are moving with respect to an interrogator, or may be located in fixed positions in the path of a moving interrogator. The interrogator, which is stationary if the tag is moving, sends a continuous RF signal to the remote tag. The tag backscatter-modulates the received RF signal with data temporarily or permanently stored in the tag, including, for example, data associated with the object to which the tag is attached, for example, its identity or contents. The system of this invention uses a very efficient encoding technique for the data which is backscatter-modulated by the tag and received by the interrogator. The backscatter-modulated signals are made up of signals of first and second frequencies f.sub.1 and 2f.sub.1, respectively, where the second frequency 2f.sub.1 is twice the first frequency f.sub.1, and where one of the two binary bits ONE and ZERO is represented by one-half period of a signal of the first frequency followed by one period of the second frequency, and the other of the two binary bits is represented by one period of a signal of the second frequency followed by one-half period of a signal of the first frequency. The signals also contain FRAME MARKERS made up of five periods of frequency 2f.sub.1 followed by one-half period of frequency f.sub.1.