Originally Posted by
Maulet
All romance languages have certain similarities, however, spanish (more correctly castillian, for it's the language of the region of castilla, which ocupied the rest of what is now called spain), because of the influence of basque and arab, it's generally regarded as the most different of romance languages. Thus the difference between catalan, portuguese, italian, occitan or french between them is much lower than that of any of them with spanish. A most notable difference is that spanish has only 5 vowels, whilst most of the other romance languages have more vocalic sounds. For instance catalan has seven written diferent vocals, and several ways of pronouncing them. Another would be the lack of distintion b/v in spanish that exists in all other indoeuropean languages that i know.
In any case, gramatical, lexical, and pronounciational differences are big enough to consider it a different language if that was your question.