Before revision:  We simplify the study of steady states of these systems by explicit elimination of intermediate complexes  (inspired by Feliu-Wiuf'12 , 13 and Thomson-Gunawardena'09)and we  define an important subclass of MESSI systems with toric steady states . We  give for MESSI systems with toric steady states an easy algorithm to determine the capacity for multistationarity.
After revision:  We simplify the study of steady states of these systems by explicit elimination of intermediate complexes (inspired by Feliu-Wiuf'12 , 13 and Thomson-Gunawardena'09)and we define an important subclass of MESSI systems with toric steady states P\'erez Mill\'an et al. (2012) give for MESSI systems with toric steady states an easy algorithm to determine the capacity for multistationarity.