as well vs besides

as well and besides both are adverbs.

Word	Noun	Adjective	Verb	Adverb
as well	No	No	No	Yes
besides	No	No	No	Yes

As adverbs, as well and besides are synonyms defined as:

Other synonyms of as well include also, likewise, too.

as well (adverb)	besides (adverb)
in addition	making an additional point; anyway
	in addition