Insert accented string into Oracle varchar2

Hi guys , I'm fronting strange trouble (as I see). I've to insert strings into Oracle varchar2 column. Matter's c# string contains accented characters , i.e. à è . I 'm using Microsoft Data Provider for Oracle. After inserting the string , I check it into the Oracle db and I see that's filtered in its accented characters! so that 'à' is transformed into 'a' and 'è' into 'e' and so on. If I reply the insert statement directly throw SQL*Plus it executes correctly , inserting accented characters in the right way! So , I guess , matter's in my Oracle provider! I 've tried using Unicode='True' in the connection string but it didn't work! I've always passed the string to insert using OracleParameter helper , settind the OracleType.Varchar , what could I do ? any ideas out there? (Excuse for my bad english...) Kind regards , Nicola