If I am not wrong, ISIN numbers last position is a verification digit. What is the mathematical function that determines its value in function of the first 11 digits?
http://en.wikipedia.org/wiki/International_Securities_Identification_Number
The procedure for calculating ISIN check digits is similar to the "Modulus 10 Double Add Double" technique used in CUSIPs. To calculate the check digit, first convert any letters to numbers by adding their ordinal position in the alphabet to 9, such that A = 10 and M = 22. Starting with the right most digit, every other digit is multiplied by two. (For CUSIP check digits, these two steps are reversed.) The resulting string of digits (numbers greater than 9 becoming two separate digits) are added up. Subtract this sum from the smallest number ending with zero that is greater than or equal to it: this gives the check digit, which is also known as the ten's complement of the sum modulo 10. That is, the resulting sum, including the check-digit, is a multiple of 10.
They have a good example too.