Hi all Having done some further research, I suspect this is NOT a boost bug, but is a bug in the microsoft implementation of ostringstream (which I suspect is used for the string concatenation). Replacing ostringstream with boost::format has resulted in several bugs with the same general charateristics disappearing! John John Foster Senior Quantitative Credit Developer Model Development, Group Credit Portfolio & Models National Australia Bank Level 15, 500 Bourke St, Melbourne VIC 3000 Tel: +61 (0)3 8641 5228 Email: John.X.Foster@nab.com.au This e-mail is sent by or on behalf of the named sender identified above. If: (a) you do not wish to receive any e-mail marketing material from this person in the future, please forward the contents of this email to unsubscribe@nab.com.au with the word "unsubscribe" in the subject box. (b) you wish to unsubscribe from all central e-mail marketing lists used by our business, please forward the contents of this e-mail to unsubscribeall@nab.com.au with the message "unsubscribe from all central e-mail marketing lists" in the subject box. If you do not forward the contents of this e-mail with your unsubscription then it may not be able to be implemented. The information contained in this e-mail communication may be confidential. You should only read, disclose, re-transmit, copy, distribute, act in reliance on or commercialise the information if you are authorised to do so. If you are not the intended recipient of this e-mail communication, please immediately notify us by e-mail to postmaster@nab.com.au, or reply by e-mail direct to the sender and then destroy any electronic and paper copy of this message. Any views expressed in this e-mail communication are those of the individual sender, except where the sender specifically states them to be the views of a member of the National Australia Bank Group of companies. Any advice contained in this e-mail has been prepared without taking into account your objectives, financial situation or needs. Before acting on any advice in this e-mail, National Australia Bank Limited recommends that you consider whether it is appropriate for your circumstances. If this e-mail contains reference to any financial products, the National recommends you consider the Product Disclosure Statement (PDS) or other disclosure document before making any decisions regarding any products. The National Australia Bank Group of companies does not represent, warrant or guarantee that the integrity of this communication has been maintained nor that the communication is free of errors, virus or interference.