diff -wu boost-trunk/boost/math/complex/acosh.hpp boost-rbk/boost/math/complex//acosh.hpp
--- boost-trunk/boost/math/complex/acosh.hpp	2011-12-13 21:56:37.088872220 +0100
+++ boost-rbk/boost/math/complex//acosh.hpp	2011-12-13 22:58:13.448327041 +0100
@@ -24,7 +24,7 @@
    // as well as compatibility with C99.
    //
    std::complex<T> result = boost::math::acos(z);
-   if(!(boost::math::isnan)(result.imag()) && signbit(result.imag()))
+   if(!(boost::math::isnan)(z.imag()) && !signbit(z.imag()))
       return detail::mult_i(result);
    return detail::mult_minus_i(result);
 }
Common subdirectories: boost-trunk/boost/math/complex/.svn and boost-rbk/boost/math/complex//.svn