This one took the better part of an afternoon from me, but I finally found the (unexpected) culprit. In the xml documentation on page 31 it is said: The given expression between [] is converted to a LUA expression so you can use the use operators: == ~= <= >= < > not and or () In addition, = equals == and != is the same as ~=. This is not true as the following demonstrates. The use of != is as expected but the ~= leads to an error and this should not be: xml> lpath > error in expression: number(@atta) ~= 1 => expr.number((ll.athttp://ll.at and ll.athttp://ll.at['atta'])) ~== 1 Hans van der Meer % failure of test on inequality. \startxmlsetups demo:comparefail \xmlsetsetup{#1}{root|node}{demo:comparefail:*} \stopxmlsetups \xmlregisterdocumentsetup{demo}{demo:comparefail} \startxmlsetups demo:comparefail:root compare \type{!=}\crlf \xmlfilter{#1}{/[number(@atta) != 1]/command(demo:comparefail:node)} compare \type{~=}\crlf \xmlfilter{#1}{/[number(@atta) ~= 1]/command(demo:comparefail:node)} \stopxmlsetups \startxmlsetups demo:comparefail:node node: attribute atta = \xmlatt{#1}{atta}\crlf \stopxmlsetups \startbuffer[comparefail] <root> <node atta="2"/> <node atta="1"/> </root> \stopbuffer \starttext \xmlprocessbuffer{demo}{comparefail}{} Note the absence of output in the second \tex{xmlfilter}. \stoptext