If you want answers it's better to give some details about the scenario and what code you are using.
If this is a server-side transform it is because you start with one character set and then switch, often because of using Response.Write in ASP or similar which is bad practice.
If client side it's often because the encoding stated at the top of the file does not match the real encoding.
Show us how you are transforming etc.
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Joe (
Microsoft MVP - XML)