Hello, AFAIK, an expression with \primitive will become expandable if and only if the primitive is expandable. (pdfTeX: \pdfprimitive) The problem arises with \pdfoutput, a kind of count register. Therefore it isn't expandable. This doesn't hurt, if \pdfoutput is set. However it cannot get its value anymore: \gdef\pdfoutput{\primitive\pdfoutput} a) \ifcase\pdfoutput fails with ! Missing number, treated as zero. <to be read again> \primitive b) \ifcase\the\pdfoutput fails with ! You can't use `\primitive' after \the. c) \ifcase\number\pdfoutput fails with ! Missing number, treated as zero. <to be read again> \primitive d) \count0=\pdfoutput fails with ! Missing number, treated as zero. <to be read again> \primitive Yours sincerely Heiko