Hi, >> These SDIO cards are working on 13 MHz frequency. Make sure you have right application for that. The company I'm working for is using 915 MHz devices, because all the apps for Walmart and DOD and the rest suppose to work on that frequency. Also there are different classes and generations or RFID tags and readers that don't work with each other. << The article that I pointed to illustrates the various frequency ranges and techniques. Near Field Communications (of which RFID is the largest segment) utilizes quite a variety of "things." My interest in these cards is educational, not (as of today) practical. So, they appeal because of cost. However, your caution is well taken. One needs to understand the environment where they will be applied to make sure that there is a match. Dick Richard Grier (Microsoft MVP - Visual Basic) Hard & Software 12962 West Louisiana Avenue Lakewood, CO 80228 303-986-2179 (voice) 303-593-9315 (fax) Author of Visual Basic Programmer's Guide to Serial Communications, 4th Edition ISBN 1-890422-28-2 (391 pages) published July 2004.For faster service, contact the publisher at http://www.mabry.com/vbpgser4<http://www.mabry.com/vbpgser4>.