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? PS: I know the mini is european but to me that does not justify the difference...
Both are european-style! What sort of differences in option prices are you talking about? If you can see the disparity then why not just make the trade and see if you can arb it.