Flash/Flex	Java
Number	any of the Java numeric types
int/uint *	any of the non-floating point Java numeric types
String	String
Boolean	Boolean/boolean
Array	List
Object	org.red5.io.utils.ObjectMap
Date	java.util.Date
XML	org.w3c.dom.Document
ByteArray *	org.red5.io.amf3.ByteArray
IExternalizable *	org.red5.io.amf3.IExternalizable
ArrayCollection *	org.red5.compatibility.flex.messaging.io.ArrayCollection
ObjectProxy *	org.red5.compatibility.flex.messaging.io.ObjectProxy
Null	null
custom class **	custom class

Please note that Red5 performs automatic parameter conversion, e.g. if you pass a number to a method that takes a String as parameter, it is automatically converted.

Java	Flash/Flex
any of the Java numeric types	Number
any of the non-floating point Java numeric types	int/uint *
String	String
boolean/Boolean	Boolean
List/Set	Array
Map	Array or Object, depends on the Map contents
any objects	Object
org.red5.io.utils.ObjectMap	Object
java.util.Date	Date
org.w3c.dom.Document	XML
org.red5.io.amf3.ByteArray	ByteArray *
org.red5.io.amf3.IExternalizable	IExternalizable *
org.red5.compatibility.flex.messaging.io.ArrayCollection	ArrayCollection *
org.red5.compatibility.flex.messaging.io.ObjectProxy	ObjectProxy *
null	Null
custom class	custom class **

^* = Only available in Flash Player 9 or newer (AMF3)
^** = You can map the class to serialize to in Red5 by adding "[RemoteClass(alias="package.to.RemoteClass")]" above your Flex class definition.