> No, I'm already using PreparedStatements, that is I use the setXXX()
> family of methods to set data for the query.

Then it should not happen. Can you prepare a standalone test case that
reproduces this problem? You can extend
org.firebirdsql.jdbc.TestFBEncodings test case (it already contains
tests for WIN1251, WIN1252, and UNICODE_FSS encodings.

Best regards,
Roman Rokytskyy