A method for modification of data in a card transaction system having a memory card and a reader for reading the card. The card has a first memory (RAM) and a second memory (EEPROM) with data locations occupied by data recordings. Each transaction involves the modification of at least one of the data locations or the addition of a new recording. The method reads the address of a free location from a previous control register located in a first fixed location in the EEPROM memory, writes the new modified recording or addition in the free location, repeat the steps for each new recording to be modified or added, and writes in a second fixed location in the EEPROM memory a new control register containing the addresses of free locations within the EEPROM memory to use in the next transaction.