Owing to the time-varying nature of the HF channel, error correcting codes of a fixed rate provide unnecessary correction power for much of the time, with consequently low data rates. The purpose of an adaptive coding scheme is to permit only the necessary degree of error correction to be applied to the transmitted information according to the channel conditions. Such a coding scheme, based on product codes, is introduced, and the improvement, compared to similar fixed rate codes, is demonstrated. A method of improving the code rate further is described, and the results of this and an interleaving procedure are also given.