Being RESTful and the data encoding used are largely orthogonal aspects of an API.

Wikipedia RESTful article says:

> The formal REST constraints are as follows:[10]

> Client/Server – Clients are separated from servers by a well-defined interface

> Stateless – A specific client does not consume server storage when the client is "at rest"

> Cache – Responses indicate their own cacheability

> Uniform interface

> Layered system – A client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way

The key line is actually this:

>An application that adheres to the REST architectural constraints may be informally described as RESTful, although this term is more commonly associated with the design of HTTP-based APIs and what are widely considered best practices regarding the "verbs" (HTTP methods) a resource responds to, while having little to do with REST as originally formulated—and is often even at odds with the concept.