Sharing Encrypted (TripleDES) Token between PHP & .NET

Can anyone shed any light on why when I encrypt a string $str using Initialisation Vector $iv TripleDES (ECB mode) I get different results on .NET platform when compared to php? I am attempting to interface to a third party (.NET) host and we need to be able to both create shared secret tokens but we keep getting different results which are unusable... Any thoughts or anybody got any experience trying this? Much appreciated Skex