REAKSIYaHOI MUSALSAL, reaksiyahoi murakkabi khimiyaviero guyand, ki dar jarayoni onho paydoishi (regenerasiyai) har yak zarrai faol boisi tabdili miqdori ziyodi molekulahoi avvala (silsila) megardad. Masalan, molekulahoi khlori omekhtai khloru gidrogen az tasiri rushnoi ba atomhoi ozod judo meshavand: S12 2S1. Dar navbati khud atomhoi S1 bo molekulahoi N2 ba reaksiya daromada atomi gidrogenu 11S1 hosil mekunand: S1* + N2 NS1 + N*. Bad az on atomhoi N* ba molekulahoi S12 tasir karda, boz atomhoi SG-fO ba vujud meorand: N* + S12 -*■ NS1 + SG. Hamin tavr reaksiyaho pai ham muttasil davom mekunand. Dar vaqti sarf shudani yak kvant rushnoi hangomi chunin Reaksiyahoi musalsal taqriban sad hazor molekulai NS1 hosil meshavad. Dar Reaksiyahoi musalsali khimiyavi (suzish, polimerizatsiya va gayra) radikalhoi ozod, atomho va molekulahoi angezanda zarrahoi faol meboshand. Dar reaksiyai yadroii musalsal boshad zarrahoi faol neytronhoand. Dar ibtidoi Reaksiyahoi musalsal az tasiri rushnoi, afkanishoti ionizonanda, harorati baland razryadi elektri va inchunin az ishtiroki angezahoi gunogun radikalhoi ozod (yo khud zarrahoi faol) hosil meshavand. Reaksiyahoi musalsali khimiyavi du khel meshavand: doroi silsilai shokhaband va doroi silsilai beshokhaband. Reaksiyai fotokhimiyavii bayni khloru gidrogen, ki dar bolo ovardem, misoli Reaksiyahoi musalsali doroi silsilai beshokhaband meboshad. Hangomi Reaksiyahoi musalsali doroi silsilai shokhaband az tasiri yak radikali ozod du yo se radikali nav hosil shuda, silsilai asosii reaksiyai doroi shokhabandro tashkil mekunand. Reaksiyai oksidshavii gidrogen dar sharoiti muayyan namunai Reaksiyahoi musalsali doroi silsilai shokhaband ast:
- ,N* + 02 ON° + 0*
- ON* + N2 -► N20 + N°
- O* + N2 ON* + N* va r.
Yane dar barobari radikalhoi N° va ON*, ki silsilai asosi (be shokhaband)-ro tashkil menamoyand, atomi 0* niz ba vujud omada, ba re- aksiya dokhil megardad. Dar natijai reaksiya (3) radikalhoi ilovagii N* va ON*, ki ibtidoi silsilahoi navand, hosil meshavand. Dar tad- qiqi ilmii Reaksiyahoi musalsali khimiyavi hissai N. N. Semyonov va S. Khinshelvud kalon ast (soli 1958 ba onho Mukofoti nobeli doda shud).