I am trying to construct an xpath expression which will select the second matching node of a nodelist.

I have:
//org[@department='Foo'][2]

I expected that to select all //org nodes where @department = 'Foo' and then give me the second within that set.

however this is behaving as
//org[@department='Foo'/self::node()[position() = 2] which will always be empty.

No, it is actually behaving as

/descendant-or-self::node/(child::org[@department='Foo'][2])

that is, it will select every org that is the second child of its parent
that has @department foo.

You probably want (//org)[@department='Foo'][2]

Grateful,
Geoff