I always used POST but according to the W3C standard, SOAP supports both POST and GET methods.
Edit: After some research, it seems that it's not completely true, as you can see here. It is theoretically possible to use GET because POST and GET are methods of HTTP transport protocol and SOAP can be used over HTTP.
But as you know, GET includes the request in the query string. SOAP requests (XML messages) are usually too complex and verbose to be included in the query string, so almost every implementation (for example JAX-WS) supports only POST.
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