diff --git a/src/libespeak-ng/dictionary.c b/src/libespeak-ng/dictionary.c
index 8832f6b4..8d50e7b9 100644
--- a/src/libespeak-ng/dictionary.c
+++ b/src/libespeak-ng/dictionary.c
@@ -32,6 +32,7 @@
 #include <espeak-ng/encoding.h>
 
 #include "readclause.h"
+#include "synthdata.h"
 
 #include "speech.h"
 #include "phoneme.h"
diff --git a/src/libespeak-ng/phonemelist.c b/src/libespeak-ng/phonemelist.c
index a304b8c5..c3af9ea6 100644
--- a/src/libespeak-ng/phonemelist.c
+++ b/src/libespeak-ng/phonemelist.c
@@ -30,6 +30,7 @@
 #include <espeak-ng/encoding.h>
 
 #include "phonemelist.h"
+#include "synthdata.h"
 
 #include "phoneme.h"
 #include "voice.h"
diff --git a/src/libespeak-ng/setlengths.c b/src/libespeak-ng/setlengths.c
index 923d4456..0affb765 100644
--- a/src/libespeak-ng/setlengths.c
+++ b/src/libespeak-ng/setlengths.c
@@ -30,6 +30,7 @@
 
 #include "readclause.h"
 #include "setlengths.h"
+#include "synthdata.h"
 
 #include "phoneme.h"
 #include "voice.h"
diff --git a/src/libespeak-ng/synth_mbrola.c b/src/libespeak-ng/synth_mbrola.c
index 534b0163..0b226ad0 100644
--- a/src/libespeak-ng/synth_mbrola.c
+++ b/src/libespeak-ng/synth_mbrola.c
@@ -33,6 +33,7 @@
 #include <espeak-ng/encoding.h>
 
 #include "readclause.h"
+#include "synthdata.h"
 
 #include "speech.h"
 #include "phoneme.h"
diff --git a/src/libespeak-ng/synthdata.c b/src/libespeak-ng/synthdata.c
index 019c2f6f..25fde001 100644
--- a/src/libespeak-ng/synthdata.c
+++ b/src/libespeak-ng/synthdata.c
@@ -32,6 +32,7 @@
 #include <espeak-ng/encoding.h>
 
 #include "readclause.h"
+#include "synthdata.h"
 
 #include "error.h"
 #include "speech.h"
diff --git a/src/libespeak-ng/synthdata.h b/src/libespeak-ng/synthdata.h
new file mode 100644
index 00000000..83b3e448
--- /dev/null
+++ b/src/libespeak-ng/synthdata.h
@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2005 to 2015 by Jonathan Duddington
+ * email: jonsd@users.sourceforge.net
+ * Copyright (C) 2015-2018 Reece H. Dunn
+ * Copyright (C) 2018 Juho Hiltunen
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see: <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef ESPEAK_NG_SYNTHDATA_H
+#define ESPEAK_NG_SYNTHDATA_H
+
+#include "translate.h"
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+// forward declare these to avoid including the whole synthesize.h
+struct PHONEME_LIST;
+struct PHONEME_DATA;
+struct WORD_PH_DATA;
+
+void InterpretPhoneme(Translator *tr, int control, PHONEME_LIST *plist, PHONEME_DATA *phdata, WORD_PH_DATA *worddata);
+void InterpretPhoneme2(int phcode, PHONEME_DATA *phdata);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+
diff --git a/src/libespeak-ng/synthesize.c b/src/libespeak-ng/synthesize.c
index e778ae35..f12895e0 100644
--- a/src/libespeak-ng/synthesize.c
+++ b/src/libespeak-ng/synthesize.c
@@ -34,6 +34,7 @@
 
 #include "intonation.h"
 #include "setlengths.h"
+#include "synthdata.h"
 
 #include "phoneme.h"
 #include "voice.h"
diff --git a/src/libespeak-ng/translate.h b/src/libespeak-ng/translate.h
index d1e7f89b..4ac71987 100644
--- a/src/libespeak-ng/translate.h
+++ b/src/libespeak-ng/translate.h
@@ -749,8 +749,6 @@ int TranslateWord(Translator *tr, char *word1, WORD_TAB *wtab, char *word_out);
 void TranslateClause(Translator *tr, int *tone, char **voice_change);
 
 void SetVoiceStack(espeak_VOICE *v, const char *variant_name);
-void InterpretPhoneme(Translator *tr, int control, PHONEME_LIST *plist, PHONEME_DATA *phdata, WORD_PH_DATA *worddata);
-void InterpretPhoneme2(int phcode, PHONEME_DATA *phdata);
 char *WritePhMnemonic(char *phon_out, PHONEME_TAB *ph, PHONEME_LIST *plist, int use_ipa, int *flags);
 
 extern FILE *f_trans; // for logging