Although it has long been known that the glass transition temperature (T-g) of nylons changes significantly with moisture content, literature to date has lacked reliable measurements of this type, primarily due to the changes in moisture during the T-g scanning experiments. Introduction of the new dynamic DSC technique has allowed us to quantify the dependence of T-g on % relative humidity for nylon 6 and nylon 66. By eliminating the possibility of moisture pickup, we have been able to separate the effect of crystallinity as well as orientation on the T-g of nylon 6. The methodology described here should be applicable to other moisture sensitive polymers as well, e.g., poly(vinyl alcohol).