The invention relates to particular substituted heterocycle fused gamma-carbolines, of Formula I as described herein, their prodrugs, in free, solid, pharmaceutically acceptable salt and/or substantially pure form as described herein, pharmaceutical compositions thereof, and methods of use in the treatment of diseases involving 5-HT2A receptor, serotonin transporter (SERT) and/or pathways involving dopamine D2 receptor signaling systems, such as diseases of the central nervous system.