Although the specifications indicate 32-bit support only, it appears the latest driver (provided on a Mini CD with the adapter or for download) supports 64-bit machines.
Specifications:
http://www.cablesunlimited.com/Specifications/specI3274.pdfDriver Downloads:
http://www.ftdichip.com/Drivers/VCP.htmI suggest you contact a Cables Unlimited representative to verify this.
One other thing, it’s labeled a USB to RS232 Serial Adapter on the package, but the driver CD (which supports multiple products) refers to it as a USB to RS232 Serial Converter.