When I look on the CBOE's website concerning the mini-spx options, it really says they are exacty 1/10 of the regular spx contract. The live quotes of both of them do confirm that. http://www.cboe.com/micro/xsp/introduction.aspx My problem is with the apparent disparity in the way the options of the corresponding strike and month are priced. There does - and quite often - seem to be a possibility of arbitrage between the 2 of them and every theory or book indicates these would not last. Is there something I am missing about these guys?