diff --git a/include/reaction.h b/include/reaction.h
new file mode 100644
index 0000000000000000000000000000000000000000..dd2b9f3e6a666a7a99dcf165f673674142fcc2a6
--- /dev/null
+++ b/include/reaction.h
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+/** @file reaction.c
+ *  @brief Header file to define reactions.
+ */
+#ifndef _REACTION_H_
+#include "react_table.h"
+
+typedef struct reaction_t reaction_t;
+
+/* Maximum number of reactants that can enter into a reaction. */
+#define REACTION_MAX_IN 4
+#define REACTION_MAX_OUT 6
+
+struct reaction_t {
+  /* Photoionization is an special reaction where all the other parameters
+   * are ignored. Be careful to put photoionization in the correct position
+   * of the reaction list.
+   */
+  int is_photo;
+
+  /* Number of species in and out. */
+  int nin, nout;
+
+  int input[REACTION_MAX_IN];
+  int output[REACTION_MAX_OUT];
+
+  const char *inname[REACTION_MAX_IN];
+  const char *outname[REACTION_MAX_OUT];
+
+
+  void (*f) (double *in, int nin, double *out, int nout, double k, double dens, react_table *rt);
+
+  /* For reactions where k(E) is given by a reaction table. Such reactions
+   * require 'f' to be f_react_table. 'k' is ignored for these reactions.
+   * 'rt' is initialized with 'NULL', while 'tablefile' contains the filename
+   * with the reaction table.
+   *
+   * This file is read in react_add().
+   */
+  react_table *rt;
+
+  char *tablefile;
+
+  double k;
+
+  /* This allows us to define many simultaneous reactions. */
+  reaction_t *next;
+};
+
+extern reaction_t *reactions_list;
+extern reaction_t *reaction_index[];
+
+#define _REACTION_H_
+#endif