diff --git a/rs.rkt b/rs.rkt
index b49841c..9daabc5 100644
--- a/rs.rkt
+++ b/rs.rkt
@@ -4,7 +4,7 @@
   (require typed/graph "utils.rkt" "dynamics.rkt")
 
   (provide
-   Species (struct-out reaction) Reaction
+   Species (struct-out reaction) Reaction ReactionSystem
    make-reaction enabled?)
 
   (module+ test
@@ -40,6 +40,8 @@
                              (set 'a 'b 'c)))
       (check-false (enabled? (make-reaction '(a b) '(c d) '())
                              (set 'b 'e)))))
+
+  (define-type ReactionSystem (HashTable Symbol Reaction))
   )
 
 (require graph "utils.rkt" "generic.rkt")
diff --git a/scribblings/rs.scrbl b/scribblings/rs.scrbl
index ed5d439..8b209b9 100644
--- a/scribblings/rs.scrbl
+++ b/scribblings/rs.scrbl
@@ -73,6 +73,14 @@ reactants are in the set and none of its inhibitors are.
           (set 'b 'e))
 ]}
 
+@deftype[ReactionSystem]{
+
+A reaction system is a dictionary mapping reaction names
+(@racket[Symbol]s) to @racket[Reaction]s.
+
+}
+
+
 @section{Org-mode interaction}
 
 This section contains some useful primitives for