diff --git a/reactos/include/psdk/mmsystem.h b/reactos/include/psdk/mmsystem.h
index 4a88123e407..9caa0944096 100644
--- a/reactos/include/psdk/mmsystem.h
+++ b/reactos/include/psdk/mmsystem.h
@@ -1343,7 +1343,7 @@ typedef struct tMIXERCONTROLDETAILS_UNSIGNED {
 	DWORD dwValue;
 } MIXERCONTROLDETAILS_UNSIGNED,*PMIXERCONTROLDETAILS_UNSIGNED,*LPMIXERCONTROLDETAILS_UNSIGNED;
 
-typedef void(CALLBACK TIMECALLBACK)(UINT,UINT,DWORD,DWORD,DWORD);
+typedef void(CALLBACK TIMECALLBACK)(UINT,UINT,DWORD_PTR,DWORD_PTR,DWORD_PTR);
 
 typedef TIMECALLBACK *LPTIMECALLBACK;
 
@@ -1471,11 +1471,11 @@ typedef struct _MMCKINFO {
 typedef const MMCKINFO *LPCMMCKINFO;
 
 typedef struct tagMCI_GENERIC_PARMS {
-	DWORD dwCallback;
+	DWORD_PTR dwCallback;
 } MCI_GENERIC_PARMS,*PMCI_GENERIC_PARMS,*LPMCI_GENERIC_PARMS;
 
 typedef struct tagMCI_OPEN_PARMSA {
-	DWORD dwCallback;
+	DWORD_PTR dwCallback;
 	MCIDEVICEID wDeviceID;
 	LPCSTR lpstrDeviceType;
 	LPCSTR lpstrElementName;
@@ -1483,7 +1483,7 @@ typedef struct tagMCI_OPEN_PARMSA {
 } MCI_OPEN_PARMSA,*PMCI_OPEN_PARMSA,*LPMCI_OPEN_PARMSA;
 
 typedef struct tagMCI_OPEN_PARMSW {
-	DWORD dwCallback;
+	DWORD_PTR dwCallback;
 	MCIDEVICEID wDeviceID;
 	LPCWSTR lpstrDeviceType;
 	LPCWSTR lpstrElementName;
@@ -1491,13 +1491,13 @@ typedef struct tagMCI_OPEN_PARMSW {
 } MCI_OPEN_PARMSW,*PMCI_OPEN_PARMSW,*LPMCI_OPEN_PARMSW;
 
 typedef struct tagMCI_PLAY_PARMS {
-	DWORD dwCallback;
+	DWORD_PTR dwCallback;
 	DWORD dwFrom;
 	DWORD dwTo;
 } MCI_PLAY_PARMS,*PMCI_PLAY_PARMS,*LPMCI_PLAY_PARMS;
 
 typedef struct tagMCI_SEEK_PARMS {
-	DWORD dwCallback;
+	DWORD_PTR dwCallback;
 	DWORD dwTo;
 } MCI_SEEK_PARMS, *PMCI_SEEK_PARMS,*LPMCI_SEEK_PARMS;