I previously posted this to the users group (http://search.gmane.org/search.php?group=gmane.comp.lib.boost.user&query=%5Bformat%5D+%25d%2C+%25u+etc.+with+%28unsigned%29+char+types) with no response - I thought I'd try here before giving up, so sorry for repeating myself... Anyway, using %d, %u etc. with char types prints the character rather than the integral value. I've had a quick hack around with the code, adding the following to feed_args.hpp: template< class Ch, class Tr> inline void put_last( BOOST_IO_STD basic_ostream<Ch, Tr> & os, const char& x ) { if (os.flags() & (std::ios_base::dec | std::ios_base::hex | std::ios_base::oct)) os << static_cast<int32_t>(x); else os << x ; } (ditto for unsigned char) and modding internals.hpp: template<class Ch, class Tr> inline void stream_format_state<Ch,Tr>:: reset(Ch fill) { // set our params to standard's default state. cf § 27.4.4.1 of the C++ norm width_=0; precision_=6; fill_=fill; // default is widen(' '), but we cant compute it without the locale flags_ = std::ios_base::skipws; // the adjust_field part is left equal to 0, which means right. exceptions_ = std::ios_base::goodbit; rdstate_ = std::ios_base::goodbit; } (i.e. remove std::ios_base::dec from flags_) 'works' (vc71) for the very few limited cases I've tried. I'm sure that's horribly broken for any number of reasons, but I'm more interested in whether this is considered a problem or not? It seems a shame to have to cast arguments to format if they happen to be char and %d is specified in the format string (also obviously making writing generic code more difficult!). Thanks in advance Luke Elliott.