The invention relates to particular substituted heterocycle fused gamma-carbolines, the compounds of formula I: 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 central nervous system (CNS) diseases involving the 5-HT2A receptor, the serotonin transporter (SERT), pathways involving the dopamine D1 and D2 receptor signaling system, and/or the μ-opioid receptor.