diff --git a/rs.rkt b/rs.rkt
index 8f17c05..50e9e53 100644
--- a/rs.rkt
+++ b/rs.rkt
@@ -8,7 +8,7 @@
  ;; Structures
  reaction
  ;; Type names
- Species
+ Species ReactionSystem
  ;; Functions
  enabled?)
 
@@ -31,3 +31,7 @@
 (define/match (enabled? r s)
   [((reaction r i p) s)
    (and (subset? r s) (set-empty? (set-intersect i s)))])
+
+;;; A reaction system is a dictionary mapping reaction names to
+;;; reactions.
+(define-type ReactionSystem (HashTable Symbol reaction))