NMR was found to be a useful technique for the analysis of impurities and excipients in Ecstasy tablets and, with further development, may be used quantitatively for determining the percentage carbon which is MDMA. Excipients detected in the analysis included lactose, cellulose, stearate salts, sucrose, starch, polyvinylpyrrolidone, and sodium croscarmellose. Two samples contained 3,4-methylenedioxy-N-ethylamphetamine (MDEA) rather than MDMA. Some interesting conformational information was also observed. Differences in chemical shifts of C-8 and C-10 carbons for MDMA-HC in solution and solid state and in different Ecstasy samples were attributed to conformational freezing and hydrogen bonding. In the solid state, carbons C-8 and C-10 were restricted from free rotation and methyl groups at C-10 and C-11 were held only in trans-conformation unlike in solution. Results were confirmed by crystal structure analysis. When excipients capable of hydrogen bonding were physically mixed with MDMA-HC, chemical shifts of carbons C-8 and C-10 in the resulting mixture changed such that they more closely resembled shifts observed in solution. 39 references, 11 tables, and 7 figures