The present invention describes a composition comprising deuterated 13 C ethanol for use in the determination of ALDH or ADH activity in vivo or in vitro. Aldehyde dehydrogenase is an emerging drug target for the treatment of heart disease, cocaine and alcohol dependence and conditions caused by genetic polymorphisms in ALDH. The present invention also describes a method for determining ALDH or ADH activity by 13 C-MR detection.