You can dematerialise only those certificates that are already registered in your name and are in the list of securities admitted for dematerialisation at NSDL or CDSL. All the scrips included in S&P, CNX, NIFTY and BSE SENSEX have already joined NSDL / CDSL. This list has more than 4,300 companies and is steadily growing. You can get an updated list of these companies from your DP or from the official website of NSDL and CDSL.