Using Boost version 1.43 to compile my project caused the following errors to occur. There are more, but they are essentially the same: g++ -o build/ftds_com/calc/DerivedCalculationInformation.o -c -Wall -ansi -Wno-missing-braces -mwin32 -mthreads -DZAF_BOOL_DEFINED -D_WIN32_WINNT=0x0501 -D__USE_W32_SOCKETS -DWIN32_LEAN_AND_MEAN -DNOMINMAX -Iinclude -I/c/dvlp/boost_1_43_0 -I/c/dvlp/vt1413_32/include "-I/c/Program Files/IVI Foundation/VISA/winnt/include" "-I/c/Program Files/Agilent/IO Libraries Suite/include" -I/c/dvlp/zaf6.0/target/h source/ftds_com/calc/DerivedCalculationInformation.cpp In file included from source/ftds_com/calc/DerivedCalculationInformation.cpp:27: include/avgcalc.h:67: error: conflicting declaration 'mpl_assertion_in_line_67' include/addcalc.h:67: error: 'mpl_assertion_in_line_67' has a previous declaration as '<anonymous enum> mpl_assertion_in_line_67' In file included from source/ftds_com/calc/DerivedCalculationInformation.cpp:35: include/dselcalc.h:68: error: conflicting declaration 'mpl_assertion_in_line_68' include/actual_airflow_calc.h:68: error: 'mpl_assertion_in_line_68' has a previous declaration as '<anonymous enum> mpl_assertion_in_line_68' In file included from source/ftds_com/calc/DerivedCalculationInformation.cpp:36: include/epowcalc.h:65: error: conflicting declaration 'mpl_assertion_in_line_65' include/datecalc.h:65: error: 'mpl_assertion_in_line_65' has a previous declaration as '<anonymous enum> mpl_assertion_in_line_65' In file included from source/ftds_com/calc/DerivedCalculationInformation.cpp:37: include/fsdvcalc.h:68: error: conflicting declaration 'mpl_assertion_in_line_68' include/actual_airflow_calc.h:68: error: 'mpl_assertion_in_line_68' has a previous declaration as '<anonymous enum> mpl_assertion_in_line_68' In file included from source/ftds_com/calc/DerivedCalculationInformation.cpp:38: include/gas_density_calc.h:68: error: conflicting declaration 'mpl_assertion_in_line_68' ... I was able to verify that these were caused by the BOOST_CLASS_VERSION macro appearing on the same line in two or more files included by the file being compiled. Using the BOOST_PP_COUNTER from the preprocessor library as the argument to a #line directive on the line preceding the BOOST_PP_COUNTER macro can be used as a workaround. Another was to disable the MPL assertion in the body of the macro. For class versioning to work for gcc, the BOOST_PP_COUNTER macro has to appear in a header file, and I think it is better to put it in the header files for the classes to be serialized. Here is a sample of the workaround: // the next two lines are needed to prevent compile-time errors with gcc #include BOOST_PP_UPDATE_COUNTER() #line BOOST_PP_COUNTER BOOST_CLASS_VERSION(Calculation, 0) Is there a better way to handle the problem? Is this a bug? Thanks, Carus V. (Bud) Clarke carus.v.clarke@boeing.com