XML-RPC supports the following data types:
A signed, 32-bit integer.
An ASCII string, which may contain NULL bytes. (Actually, several XML-RPC implementations support Unicode, thanks to the underlying features of XML.)
Either true or false.
A double-precision floating point number. (Accuracy may be limited in some implementations.)
A date and time. Unfortunately, since XML-RPC forbids the use of timezones, this is very nearly useless.
Raw binary data of any length; encoded using Base64 on the wire. Very useful. (Some implementations don't like to receive zero bytes of data, though.)
An one-dimensional array of values. Individual values may be of any type.
A collection of key-value pairs. The keys are strings; the values may be of any type.