Ash composition measurements of calcium carbonate and gypsum in paper is accomplished with a dual X-ray sensor system with one X-ray source that is powered at about 5.9 KV and a second X-ray source that is powered at about 4.2 KV. Corresponding detectors measure radiation from the respective X-ray sources that is emitted from the paper. Data derived from the measurements yields the gypsum and crystal water content in the paper. The dual X-ray sensor system can be employed in conjunction with infrared total moisture measurements of paper products being manufactured on a papermaking making machine, which contain gypsum and calcium carbonate, in order to correct for the gypsum crystal water effect.