Thanks Paul, So are you saying that the preprocessor should look at BOOST_PP_REPEAT_FROM_TO and begin expanding that first, in isolation from what follows next on the line i.e. the parameters that are to be passed to whatever BOOST_PP_REPEAT_FROM_TO "returns", and then reset the macro replacement stack before continuing? That would explain a lot. It would also explain why even if I modify the other code to this: #define CONCAT_1(a, b) CONCAT_1_D(a, b) #define CONCAT_1_D(a, b) a ## b #define CONCAT_2(a, b) CONCAT_2_D(a, b) #define CONCAT_2_D(a, b) a ## b #define AB(c, x, y) CONCAT_ ## c(x,y) #define CONCAT CONCAT_1 CONCAT(A, B(1, p, q)) // Not calling CONCAT_1 directly any more Expansion still stops early because after expanding CONCAT (which is not a function macro this time) and deleting the replacement stack we are still kicking the next lot of expansion off with CONCAT_1, so we cannot expand it again? Thanks for your time Paul. I tried responding personally but your mail server considers me to be a spammer ;-) Brad