Method of reinitializing dictionaries in a data transmission system using data compression having a transmit device and a receive device, and in which strings of characters have to be transmitted in a compressed form, the transmit device having a transmit dictionary storing codewords associated with the strings of characters which are transmitted instead of the strings of characters, the receive device having a receive dictionary storing codewords associated with the strings of characters, and both dictionaries being updated each time a new string of characters has to be transmitted so that the contents of the dictionaries remain identical. This method saves at least the addresses of the parts of the transmit or receive dictionary which have to be modified by a dictionary updating operation, builds a check message based upon the contents of the transmit dictionary updated by the dictionary updating operation, transmits the check message from the transmit device to the receive device, and then deletes in both dictionaries the parts which are determined by the saved addresses in the event that the check message does not correspond to the contents of the updated receive dictionary. This enables both dictionaries to be reinitialized in an intermediate state without being reset.