Why do instance variables seemingly disappear when inside a block?

Forgive me, guys. I am at best a novice when it comes to Ruby. I'm just curious to know the explanation for what seems like pretty odd behavior to me.

I'm using the Savon library to interact with a SOAP service in my Ruby app. What I noticed is that the following code (in a class I've written to handle this interaction) seems to pass empty values where I expect the values of member fields to go:

create_session_response = client.request "createSession" do
  soap.body = {
    :user => @user, # This ends up being empty in the SOAP request,
    :pass => @pass  # as does this.
  }
end

This is despite the fact that both @user and @pass have been initialized as non-empty strings.

When I change the code to use locals instead, it works the way I expect:

user = @user
pass = @pass

create_session_response = client.request "createSession" do
  soap.body = {
    :user => user, # Now this has the value I expect in the SOAP request,
    :pass => pass  # and this does too.
  }
end

I'm guessing this strange (to me) behavior must have something to do with the fact that I'm inside a block; but really, I have no clue. Could someone enlighten me on this one?